Oracle Database 10g The Complete Reference phần 6 pptx

Therefore, we first need to create an abstract datatype that has the same structure as the CUSTOMER Now, create an object view based on the CUSTOMER_TY type, while assigning OID values t

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back to CUSTOMER—you cannot record a call for a customer who does not already have a record in

CUSTOMER A single nonkey attribute, Call_Date, is created within the CUSTOMER_CALL table

create table CUSTOMER_CALL

For example, you could have the following CUSTOMER and CUSTOMER_CALL entries:

insert into CUSTOMER values

(123,'SIGMUND','47 HAFFNER RD','LEWISTON','NJ',22222);

insert into CUSTOMER values

(234,'EVELYN','555 HIGH ST','LOWLANDS PARK','NE',33333);

insert into CUSTOMER_CALL values

CUSTOMER Therefore, we must find a way to assign OID values to the records in CUSTOMER

and generate references in CUSTOMER_CALL

How to Generate OIDs

First, use an object view to assign OIDs to the records in CUSTOMER Remember that OIDs are

assigned to records in an object table—and an object table, in turn, is based on an abstract datatype

Therefore, we first need to create an abstract datatype that has the same structure as the CUSTOMER

Now, create an object view based on the CUSTOMER_TY type, while assigning OID values

to the records in CUSTOMER:

create or replace view CUSTOMER_OV of CUSTOMER_TY

with object identifier (Customer_ID) as

658 Part V: Object-Relational Databases

ORACLE Series TIGHT / Oracle Database 10g: TCR / Loney / 225351-7 / Chapter 36

Blind Folio 36:658

P:\010Comp\Oracle8\351-7\CD\Ventura\book.vp

Composite Default screen

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select Customer_ID, Name, Street, City, State, Zip

from CUSTOMER;

The first part of this create view command gives the view its name (CUSTOMER_OV) and tells

Oracle that the view’s structure is based on the CUSTOMER_TY datatype:

create or replace view CUSTOMER_OV of CUSTOMER_TY

The next part of the create view command tells the database how to construct OID values for the rows in CUSTOMER The with object identifier clause is followed by the column to use for the

OID—in this case, the Customer_ID value This will allow you to address the rows within the

CUSTOMER table as if they were referenceable row objects within an object table

with object identifier (Customer_ID) as

The final part of the create view command gives the query on which the view’s data access

will be based The columns in the query must match the columns in the view’s base datatype

select Customer_ID, Name, Street, City, State, Zip

from CUSTOMER;

The rows of CUSTOMER are now accessible as row objects via the CUSTOMER_OV view

The OID values generated for the CUSTOMER_OV rows are calledpkOIDs, because they are

based on CUSTOMER’s primary key values Relational tables can be accessed as row objects if

you create object views for them

How to Generate References

The rows of CUSTOMER_CALL reference rows in CUSTOMER From a relational perspective, the

relationship is determined by the foreign key pointing from the CUSTOMER_CALL.Customer_ID

column to the CUSTOMER.Customer_ID column Now that the CUSTOMER_OV object view has

been created, and the rows in CUSTOMER can be accessed via OIDs, you need to create reference

values in CUSTOMER_CALL that reference CUSTOMER Once the REFs are in place, you will be

able to use the DEREF function (shown earlier in this chapter) to access the CUSTOMER data from

within CUSTOMER_CALL

The create view command for the object view of CUSTOMER_CALL is shown in the following listing It uses a new function, MAKE_REF, which is described following the listing.

create view CUSTOMER_CALL_OV as

select MAKE_REF(CUSTOMER_OV, Customer_ID) Customer_ID,

Call_Number, Call_Date from CUSTOMER_CALL;

With the exception of the MAKE_REF operation, this create view command looks like a normal create view command The MAKE_REF operation is shown in this line:

select MAKE_REF(CUSTOMER_OV, Customer_ID) Customer_ID,

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The MAKE_REF function takes as arguments the name of the object view being referenced

and the name of the column (or columns) that form the foreign key in the local table In this case,

the Customer_ID column of the CUSTOMER_CALL table references the column that is used as the

basis of OID generation in the CUSTOMER_OV object view Therefore, two parameters are passed

to MAKE_REF: CUSTOMER_OV and Customer_ID The result of the MAKE_REF operation is given

the column alias Customer_ID Since this command creates a view, the result of an operation must

be given a column alias

What does MAKE_REF do? It creates references (calledpkREFs, since they are based on primarykeys) from the CUSTOMER_CALL_OV view to the CUSTOMER_OV view You can now query

the two views as if CUSTOMER_OV were an object table and CUSTOMER_CALL_OV were a

table that contains a REF datatype that references CUSTOMER_OV

Querying the Object Views

The queries of the object views with REFs mirror the structure of the queries of table REFs You

use the DEREF function to select the value of the referenced data, as shown earlier in this chapter.

Applied to the object views, the query will be

select DEREF(CCOV.Customer_ID)

from CUSTOMER_CALL_OV CCOV

where Call_Date = TRUNC(SysDate);

DEREF(CCOV.CUSTOMER_ID)(CUSTOMER_ID, NAME, STREET, CITY, STATE,ZIP)

-CUSTOMER_TY(123, 'SIGMUND', '47 HAFFNER RD', 'LEWISTON','NJ',22222)

The query found the record in CUSTOMER_CALL for which the Call_Date value was the current

system date It then took the Customer_ID value from that record and evaluated its reference That

Customer_ID value, from the MAKE_REF function, pointed to a pkOID value in the CUSTOMER_

OV object view The CUSTOMER_OV object view returned the record whose pkOID matched

the referenced value The DEREF function then returned the value of the referenced row The query

thus returned rows from CUSTOMER even though the user only queried CUSTOMER_CALL

Object views of column objects enable you to work with tables as if they were both relationaltables and object-relational tables When extended to row objects, object views enable you to

generate OID values based on established foreign key/primary key relationships Object views

allow you to continue to use the existing constraints and standard insert, update, delete, and

select commands They also allow you to use OOP features such as references against the object

tables Object views thus provide an important technological bridge for migrating to an OOP

database architecture

As described earlier in this chapter, Oracle performs joins that resolve the references defined

in the database When the referenced data is retrieved, it brings back the entire row object that

was referenced To reference the data, you need to establish pkOIDs in the table that is the

“primary key” table in the relationship, and use MAKE_REF to generate references in the table

that is the “foreign key” table in the relationship You can then work with the data as if it were

stored in object tables

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Object PL/SQL

PL/SQL programs can use the abstract datatypes you have created Whereas earlier versions of PL/SQL

could only use the Oracle-provided datatypes (such as DATE, NUMBER, and VARCHAR2), you

can now use your own user-defined datatypes as well The result is the merging of SQL, procedural

logic, and OOP extensions—a combination referred to asobject PL/SQL

The following anonymous PL/SQL block uses object PL/SQL concepts The CUSTOMER_TYabstract datatype is used as the datatype for theCust1 variable, and its value is populated by a

query of the CUSTOMER_OV object view

TheCust1 variable value is selected from the CUSTOMER_OV object view (created in the

preceding section of this chapter) The VALUE function is used to retrieve the data in the structure

of the abstract datatype Since the data will be selected in the format of the abstract datatype,

you need to use the CUSTOMER_OV object view created on the CUSTOMER table

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This is a deliberately simple example, but it shows the power of object PL/SQL You can useobject PL/SQL anywhere you use abstract datatypes Your PL/SQL is thus no longer bound to the

Oracle-provided datatypes, and may more accurately reflect the objects in your database In this

example, an object view was queried to illustrate that the queries can access either column objects

or row objects You can then select the attributes of the abstract datatype and manipulate or display

them If you have defined methods for the abstract datatype, you can apply them as well

For example, you can call the datatype’s constructor methods within your PL/SQL blocks Inthe following example, a variable namedNewCust is defined using the CUSTOMER_TY datatype

TheNewCust variable is then set equal to a set of values using the CUSTOMER_TY constructor

method TheNewCust variable’s set of values is then inserted via the CUSTOMER_OV object view.

You can see the result of the insert by querying CUSTOMER_OV:

select Customer_ID, Name from CUSTOMER_OV;

datatypes, you will need to define map or order methods for the datatypes This capability allows

you to further extend object PL/SQL—you define the datatypes and the functions at the database

level, and they are callable within any of your PL/SQL programs Object PL/SQL represents a

significant enhancement over traditional PL/SQL

Objects in the Database

The features available in Oracle—column objects, row objects, and object extensions to PL/SQL—

enable you to implement objects in your database without sacrificing the investment you have

already made in analysis and design You can continue to create systems based on relational

design techniques and tune them based on relational access methods The tools that Oracle

provides allow you to create an OOP layer above your relational tables Once you have that

layer in place, you can access the relational data as if it were stored in a fully OOP database

Having an OOP layer allows you to realize some of the benefits of an OOP system, such asabstraction and encapsulation You can apply the methods for each abstract datatype across a set

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of consistently implemented objects, and benefit from object reuse and standards enforcement.

At the same time, you can benefit from Oracle’s relational features The ability to use both relational

and object technology within an application lets you use the proper tool for the proper job within

the database

Note that implementing object-relational features may force you to change your approach

to data relationships and the tools you use to access the data If you use row objects, the

relationships among tables are among row objects, not their data—possibly resulting in dangling

REFs Additionally, unless you use object views you will need to use new SQL syntax for queries

and DML operations

When implementing the object portion of an object-relational database, start by defining theabstract datatypes that are the core components of your business Every object-relational feature,

whether it relates to column objects or row objects, is based on an abstract datatype The better

you have defined your datatypes and their methods, the better you will be able to implement objects

If necessary, nest objects so that you can have multiple variations of the same core datatype The

result will be a database that is properly designed to take advantage of both relational and

object-oriented features

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VI

Java in Oracle

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37

An Introduction

to Java

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668 Part VI: Java in Oracle

I n this chapter, you will see an overview of Java as it relates to Oracle databaseapplications There are numerous uses of Java beyond Oracle applications, and there

are many features of the language that will not be used by most Oracle developers

The goal of this chapter is to provide developers who have a background in SQL andPL/SQL with an understanding of the Java language structures This is not an exhaustivereview of Java (numerous books are available that accomplish that goal) but rather a short overview

of the Java language components most commonly used by Oracle developers

Although in many cases PL/SQL and Java correlate well to each other, there are significantdifferences in terminology and usage That shouldn’t be surprising because the two programming

languages were developed independently Furthermore, PL/SQL’s object oriented capabilities

were not part of its initial implementation, whereas Java has been object oriented since its inception

To use Java effectively, you need to take a different approach than the one you may have taken

in the past with PL/SQL

Chapter 29 presented an introduction to the PL/SQL language structures and flow control

This chapter mirrors that approach: You will see the structures used by Java and the basic

flow-control methods provided Where applicable, you will also see the matching PL/SQL structures

You should be familiar with PL/SQL (Chapter 29); packages, functions, and procedures (Chapter 31);

and abstract datatypes and methods (Chapter 33) before reading this chapter

Java vs PL/SQL: An Overview

In comparing PL/SQL to Java, you get no further than the basic PL/SQL structure—a block—before

you encounter a significant difference in terminology between the two languages In PL/SQL, ablock

is a structured piece of code that has a Declarations section, an Executable Commands section, and

an Exception Handling section For a PL/SQL block, the structure may be represented as follows:

In Java, the termblock refers to a much smaller subset of code In Java, a block is a collection

of statements enclosed by curly braces, { and } For example, the following pseudo-code contains

two Java blocks:

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A PL/SQL procedure is bound by a begin and end, whereas a Java method is bound by opening

and closing braces These braces are also utilized to isolate individual blocks of scoped code, or

as indicators of the opening and closing of conditional segments

A second major difference between PL/SQL and Java is in the declaration of variables In aPL/SQL block, you define your variables before you begin the Executable Commands section

In a Java program, you can define variables where they are needed within the program A Java

program has no Declarations section comparable to the one used in PL/SQL blocks

Throughout this chapter, you will see a number of other differences between PL/SQL and Java

It’s important to remember that Java does not replace PL/SQL within your applications—you can

continue to use PL/SQL for Oracle applications For data-retrieval operations, you should test the

performance of PL/SQL and Java before deciding on a technical direction to follow

Getting Started

To use the examples in this section, you need a copy of the Java Development Kit (JDK), which is

available for free download from http://java.sun.com/ You need to install the JDK on the server on

which you will be running the Java programs As of the time of this writing, JDK 1.4 is the most

recent production release; most development efforts use JDK 1.3.1 At a minimum, you should use

JDK 1.2.2

NOTE

The JDK is installed as part of the Oracle Database 10g installation

Be sure to install both the database kernel software and the softwarethat comes on the Companion CD

Declarations

Within Java, you can declare variables as needed A variable has a name and a datatype, and it’s

used to store a data value during the program’s execution A variable also has a scope, allowing

it to be publicly accessible or private—similar to the manner in which procedures within packages

can have private variables

Here are examples of Java variable declarations:

char aCharVariable = 'J';

boolean aBooleanVariable = false;

By convention, Java variables always start with a lowercase letter, as shown in this example,where the datatypes are CHAR and BOOLEAN, and each variable is assigned an initial value

Note that the assignment character in Java is =, as opposed to := or => in PL/SQL Each declaration

is terminated with a semicolon

The available primitive datatypes in Java are listed in Table 37-1

In addition to the primitive datatypes listed in Table 37-1, you can use reference datatypes,which are based on the contents of variables You may note that there is no primitive datatype for

variable-length character strings—Java provides a String class for that purpose Classes are discussed

later in this chapter

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Executable Commands

You can assign values to variables via the use of expressions and statements The arithmetic operators

supported by Java include the following:

In addition to these operators, you can use Java’s unary operators to simplify your coding

For example, you can increment variables via the ++ operator, as shown here:

Datatype Description

Byte Byte-length integer

Short Short integer

Long Long integer

Float Single-precision floating-point real number

Double Double-precision floating-point real number

Char A single Unicode character

Boolean A Boolean value, true or false

TABLE 37-1. Primitive Java Datatypes

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In PL/SQL, that would have been written as follows:

aLoopCounter := aLoopCounter +1;

If the ++ operator is placed in front of the variable name, it is apre-increment operator ratherthan apost-increment operator:

int anotherCounter = ++aLoopCounter;

In this example, the variableanotherCounter is set to the incremented value of aLoopCounter

By contrast,

int anotherCounter = aLoopCounter++;

setsanotherCounter to the value of aLoopCounter before it is incremented You can decrement a

variable’s value via the unary operator:

You can perform similar combinations using /=, %=, +=, and –=

If you perform multiple arithmetic operations, you should use parentheses to clearly indicatethe order in which the operations should be evaluated, as shown in the following example:

aNumberVariable = (aNumberVariable*2) +2;

Terminating these operations with semicolons makes themstatements—complete units ofexecution

Conditional Logic

You can evaluate variables by using expressions and statements To do so, you may use one or

more of the fundamental classes provided as part of Java A full listing of those methods is beyond

the scope of this book; see Sun’s Java documentation site or one of the many Java books available

for the functionality currently provided

NOTE

The number of supplied classes usually changes dramatically witheach release of Java

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In the following listing, the Character.isUpperCase method is used to evaluate theaCharVariablevariable declared earlier:

if (Character.isUpperCase(aCharVariable)) {

some commands to execute

}

If theaCharVariable value is uppercase, the commands in the following block will be executed;

otherwise, flow will continue to the next part of the program

The following is the general syntax for if clauses:

if (expression) {

statement

}

Notice the lack of a then clause If the expression being evaluated is true, the following block is

automatically executed Also, note the lack of an end if clause Java blocks are simply terminated

by the closing brace of the statement

You can also have else clauses to evaluate different conditions, as shown in the following listing:

In addition to supporting if clauses for conditional logic, Java features a switch clause Used

in conjunction with its break clause and statement labels, the switch clause can approximate the

functionality of goto clauses (which Java does not support) First, let’s look at a simple switch

example Using the case statement, multiple values of theaMonth variable are evaluated Depending

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on theaMonth value, the text name for the month is displayed, and the processing leaves the

switch block by means of a break.

In Java, the arguments of a case statement must be integers.

This kind of code is much more complex to write than a simple TO_CHAR, but it illustrates the switch usage The switch operator takes theaMonth variable as its input and then evaluates

its value If the value matches a case value, the System.out.println method is called to print the

month’s name, and processing control leaves the switch block by means of a break If the value

does not match one of the case clauses, the default option is executed.

Where does control go? By default, it goes to the next section of the program However, youhave the option of creating labels for sections of your code and passing the names of those labels

to the break clause The following listing shows an example of a label and a break clause:

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assigned a value of 2, so the program will display the word “February” and will then branch over

to the section of code identified by the someotherlabel label If theaMonth value had been 1, the

if clause at the beginning of the code listing would have been reexecuted.

Java also supports aternary operator—an operator that takes three operands and whose function

is similar to that of DECODE Acting as an inline if-else combination, the ternary operator evaluates an

expression If the expression evaluates to true, the second operand is returned; otherwise, the third is

returned The syntax is as follows:

expression ? operand1 : operand2

The following listing shows a sample ternary operation:

aStatusVariable = (aCharVariable == 'V') ? "OK" : "Invalid";

In this example, the expression

(aCharVariable == 'V')

is evaluated If it is true, “OK” is returned; otherwise, “Invalid” is returned

You can use the ternary operator to simulate the use of a GREATEST function:

double greatest = (a > b) ? a : b;

In this example, the expression (a > b) is evaluated, wherea and b are the names of variables If that

expression is true,a’s value is returned; otherwise, b’s value is returned

NOTE

Java has a “greatest” method for numeric values—the Max methodwithin the java.lang.Math class The ternary example in the preceding

listing can be rewritten as double greatest = Math.Max(a, b);

You can combine multiple logic checks via the use of AND and OR operations In Java, an ANDoperation is represented by the && operator:

if (aCharVariable == 'V' && aMonth == 3) {

statement

}

The OR operation is represented by ||, as shown in the following listing:

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operator, if the first value is false, the evaluation ends at that point and the Boolean value false is

returned In the case of the OR operator, if the first expression is false, the second expression will

still be evaluated because only one of the test conditions needs to be true for the Boolean value

true to be returned If the first value of the OR operator were true, subsequent expressions would

not need to be evaluated and true would be returned

The & and | operators are bitwise AND and OR; they are not short-circuited as && and || are

All expressions will always be evaluated This functionality can be important if you are testing the

return values of method calls and you need to make sure the methods are executed

Loops

Java supports three main types of loops: WHILE loops, DO-WHILE loops, and FOR loops In this

section, you will see examples of each type of loop Because Java is not written as an extension

of SQL, it does not support Cursor FOR loops as PL/SQL does

WHILE Loops and DO-WHILE Loops

A while clause evaluates a condition; if the condition is true, the associated block of statements

is executed The syntax for a WHILE loop is of the following form:

In this example, a counter variable(aNumberVariable) is created and initialized The variable’s

value is then evaluated via the while clause If the value is less than 7, the associated statement

block is executed As part of that block, the variable is incremented When the block completes,

the variable is again evaluated and the loop continues

NOTE

For examples of WHILE loop processing, see “Classes” later in thischapter

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You could also write this as a DO-WHILE loop:

In a DO-WHILE loop, the expression’s value is not evaluated until the block has been processed

at least one time

FOR Loops

You can use a FOR loop to repeatedly execute a block of code In a FOR loop, you specify an

initial value, a termination value, and an increment for the loop counter The loop counter will

be incremented each time through the loop, by the increment you specify, until the termination

value is reached The following is the syntax for a FOR loop:

for (initialization; termination; increment) {

The for clause version for this example is much shorter than the while version In this example,

theaNumberVariable variable is declared and initialized within the FOR loop As long as the

variable’s value does not exceed 7, the block will be executed For each pass through the loop,

the variable’s value is incremented by 1 via the ++ unary operator

Within a loop, you can use the continue clause to jump to another statement within the loop.

If you just use the continue clause by itself, the process flow will jump to the end of the loop body

and evaluate the loop’s termination test If you use continue with a label (as shown previously in

the section on the switch clause), processing will continue at the next iteration of the labeled loop.

Cursors and Loops

Within PL/SQL, you can use Cursor FOR loops to iterate through the results of a query You can

use Java’s WHILE and FOR loops to simulate the functionality of a PL/SQL Cursor FOR loop The

examples provided with Oracle show how this can be accomplished Oracle provides sample

files on the Companion disc that accompanies the Oracle software installation The Companion

disc uses the Oracle Universal Installer to load software drivers and demonstration files, including

those supporting JDBC development Following installation of the Companion disc, the JDBC

examples will be in a file named demo.zip in the /jdbc/demo subdirectory under the Oracle

software home directory; when unzipped, the files referred to in this chapter will be located in

the /jdbc/demo/samples/generic subdirectory The SelectEmployee.java demo issues the following

commands:

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Statement stmt conn.createStatement();

ResultSet rset = stmt.executeQuery ("select FIRST_NAME, "

+ "LAST_NAME from EMPLOYEES");

// Iterate through the result and print the employee names

while (rset.next ()) System.out.println (rset.getString (1) + " " + rset.getString (2));

This example assumes that a connection to the database has already been established Thefirst line of the example creates a variable calledstmt, based on a connection variable called conn

(not shown in this partial listing) The second line of the example creates a variable calledrset,

based on the result set that the querystmt will return Following a comment (prefixed by // ), a

while clause fetches each row in the result set If the fetch retrieves a row from the result set,

the ENAME from that EMP row is printed; otherwise, the loop will end

From a programming perspective, you should be specific in the data returned (as in the precedingexample), specifying the column names to be returned

The JDBC (Java Database Connectivity; see Chapter 38) demos provided by Oracle showexamples of queries involving procedure calls, LOBs, and DML In most cases, your Java code

will be performing in a similar manner to the previous example: Write the process flow-control

language in Java and pass it a statement to execute Based on the outcome of that statement, you

can direct different actions to occur via the use of while, for, if, break, continue, and switch clauses,

as shown earlier in this chapter

Exception Handling

In a PL/SQL procedure, you have a single EXCEPTION block; in Java, you can have as many try/

catch blocks as you desire Java provides a robust set of error-handling routines and enables you

to create complex error-handling procedures In PL/SQL, you raise an exception; in Java, you

throw exceptions If an exception is thrown, you need to use the Java catch clause to capture

and properly process the exception

Java’s exception-handling syntax is based on three blocks: try, catch, and finally The try block includes the statements that might throw an exception The catch block immediately following the

try block associates exception handlers with the exceptions that may be thrown by the try block.

The finally block cleans up any system resources not properly cleaned up during the catch block

processing The general structure is as follows:

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For example, consider the following Java code:

In this example, a variable namedstmt is created to execute a drop table command If the

execution fails—for example, if the table does not exist—how will that error be processed? The

catch clause tells Java to handle it via an exception object called SQLException, a standard part

of Java’s SQL implementation As shown in the example, the catch clause takes two parameters

(an exception object and a variable name) and is optionally followed by a block of statements to

execute

The try block can contain multiple statements, or you can create separate try blocks for each statement In general, it is easier to manage exception handling if you consolidate your catch blocks,

so consolidating your try blocks will help you manage your code as it changes and grows The

when others clause in PL/SQL is essentially catch (Throwable th) in Java.

The finally block cleans up the state of the current code section before passing control to any subsequent parts of the program At run time, the contents of the finally block are always executed,

regardless of the outcome of the try block For example, you could use the finally block to close

a database connection

Reserved Words

The reserved words in Java are listed in Table 37-2 You cannot use these words as the names of

classes, methods, or variables

TABLE 37-2. Reserved Words in Java

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In the prior sections of this chapter, you saw the basic syntax structures for Java In this section,

you will see how to use that syntax to create and use objects A simple program for printing the

word “Oracle” is shown in the following listing:

public class HelloOracle {

public static void main (String[] args) {

more complex than

select 'Oracle' from dual;

but the Java version has features lacking in the SQL version HelloOracle is a class, and as such

its methods can be called from within other classes HelloOracle could have multiple methods;

in this example, it has only the main method

The main method’s declaration (public static void main) has a number of keywords that you

should become familiar with:

public Defining the class as public allows all classes to call this method.

static Declares that this is a class method; you use static to declare class methods An additional

option is the keyword final for methods and variables whose values cannot change (similar

to PL/SQL’s constant option).

void Specifies the return value’s datatype from the procedure Because this procedure does

not return values, its type is void.

In this example, the method is given the namemain because that will simplify running theclass’s method Now that the class has been created, you can compile and load it

TABLE 37-2. Reserved Words in Java (continued)

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First, save the program as a plain text file called HelloOracle.java Next, compile it:

and methods are defined As with abstract datatypes in Oracle, Java classes have methods associated

with them for the manipulation of their related data

Let’s consider a more complex example The following listing is a Java program that computesthe area of a circle, given a radius value as input (see Chapter 29 for the PL/SQL version):

// AreaOfCircle.java

//

public class AreaOfCircle {

public static void main(String[] args) {

try { int input = Integer.parseInt(args[0]);

double result=area(input);

System.out.println(result);

} catch (ArrayIndexOutOfBoundsException oob) { System.out.println("You did not provide the radius of the circle.");

System.out.println("Usage: java AreaOfCircle 10");

} catch (NumberFormatException nfe) { System.out.println("Enter a valid number for the radius.");

} } /// main

public static double area (int rad) {

double pi=3.1415927;

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First, note that the class and the name of the file must be the same In this case, the class isnamed AreaOfCircle, so this text is stored in a file called AreaOfCircle.java The class has two

methods, called main and area When you execute the class, the main method is automatically

executed The main method takes the string provided as input and parses it as an integer into the

input variable:

int input=Integer.parseInt(args[0]);

There are at least two possible exceptions that could be thrown when running this code Thefirst exception handled, ArrayIndexOutOfBoundsException, occurs when you try to reference a

position in an array that doesn’t exist Because the code explicitly references the first position of

the array named args (args[0]), an exception will be thrown if no data was passed to the program

The other possible error, NumberFormatException, would occur if a nonnumeric value was

passed in For example, if you ran the program by entering “java AreaOfCircle XYZ” instead of

“java AreaOfCircle 123”, the NumberFormatException would be thrown because XYZ isn’t a

valid number Both of these exceptions are handled by the two catch statements that provide the

user with useful feedback regarding the cause of the error:

catch (ArrayIndexOutOfBoundsException oob) {

System.out.println("You did not provide the radius of the circle.");

}

catch (NumberFormatException nfe) {

System.out.println("Enter a valid number for the radius.");

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public class AreaOfCircleWhile {

public static void main(String[] args) { try {

int input=Integer.parseInt(args[0]);

while (input < 7) { double result=area(input);

System.out.println(result);

input++;

} } catch (ArrayIndexOutOfBoundsException oob) { System.out.println("You did not provide the radius of the circle.");

} catch (NumberFormatException nfe) { System.out.println("Enter a valid number for the radius.");

} } /// main public static double area (int rad) { double pi=3.1415927;

double areacircle=pi*rad*rad;

return areacircle;

} /// area } /// AreaOfCircleWhile

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In this listing, the area method and the exception-handling blocks are unchanged from theprior example The change is in the main method:

While the input value is less than 7, the input value will be processed After the area for that radius

value has been calculated and printed, the input value is incremented:

The output shows the area calculations for input radius values of 4, 5, and 6

This example is not as complete as a finished program—you may need additional exceptionhandling to handle nonintegers, for example—but the structure is the important lesson to learn

from this example In addition to the methods shown in this example, you could create a constructor

for the AreaOfCircleWhile class All Java classes have constructors (see Chapter 31) to initialize

new objects based on the class The constructor’s name is the same as the name of the class If

you create a class with no constructor, Java will create a default constructor at run time Like a

method, the constructor body can contain local variable declarations, loops, and statement blocks

The constructor initializes these variables for the class

In the following chapters, you will see how to implement Java within Oracle—within storedprocedures and via JDBC Those chapters assume that you have the basic knowledge of Java provided

in this chapter For additional information about the Java language, see the Sun Microsystems

Web site and the variety of books devoted to Java

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38

JDBC Programming

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J ava Database Connectivity (JDBC) builds on the Java and programming basics describedearlier in this book The discussions and examples in this chapter assume you are

familiar with the Java syntax and structures described in Chapter 37 This chapter doesnot cover every aspect of JDBC but focuses on its basic configuration and usage

You can use JDBC to execute dynamic SQL statements in Java programs Oracleprovides sample files on the Companion disc that accompanies the Oracle software installation

The Companion disc uses the Oracle Universal Installer to load software drivers and demonstration

files, including those supporting JDBC development Following installation of the Companion disc,

the JDBC examples will be in a file named demo.zip in the /jdbc/demo subdirectory under the

Oracle software home directory; when unzipped, the files referred to in this chapter will be located

in the /jdbc/demo/samples/generic directory

The following listing is from the SelectExample.java demo file The SelectExample.java demoissues the following commands after establishing a database connection:

// Create a Statement Statement stmt = conn.createStatement ();

// Select first_name and last_name column from the employees table ResultSet rset = stmt.executeQuery ("select FIRST_NAME, "

+ "LAST_NAME from EMPLOYEES");

// Iterate through the result and print the employee names while (rset.next ())

System.out.println (rset.getString (1) + " " + rset.getString (2));

When you execute a SQL statement via JDBC—in this case, select First_Name, Last_Name from

EMPLOYEES—the SQL statement is not checked for errors until it is run The JDBC program that

contains the SQL command will compile even if the SQL statement is invalid

Getting Started

To use the examples in this section, you need a copy of the Java Development Kit (JDK), which

is installed during the Oracle Database Companion CD installation process

Additional Steps for Windows Users

If you will be accessing Oracle via JDBC, you must set the CLASSPATH environment variable to

find the classes12.zip file provided by Oracle The file is located in the /jdbc/lib directory beneath

your Oracle home directory, or it’s available for download from http://technet.oracle.com You

must also check the setting of your PATH environment variable

NOTE

Setting the CLASSPATH environment variable is not performed by thestandard Oracle installation, so you need to do it manually If the systemvariables are not properly set, you will not be able to compile Javaclasses that use Oracle’s JDBC classes The system variable settingprocedure differs by operating system version In Windows 2000, usethe System icon under the Control Panel to add and edit environmentvariable settings

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Create an environment variable named CLASSPATH if it doesn’t already exist (it won’t for aninitial Java setup) This variable must have two entries, separated by a semicolon The first entry

must be a period, which denotes the current directory The second entry must be the directory for

the classes12.zip file:

errors when executing your programs

The PATH environment variable should already be set; append the following entry to its value:

;E:\oracle\ora10\jdbc\lib\classes12.zip

Replace “E:\oracle\ora10” with your Oracle software home directory

Testing Your Connection

Oracle provides a sample program called JdbcCheckup.java that you can use to verify your JDBC

configuration This file may be in a zip file (demo.zip on the /jdbc/demo directory) or in /jdbc/demo/

samples/generic if you have unzipped it You will need to extract it before running the program Go

to the directory where the sample file is, then compile and execute the JdbcCheckup.java class:

Your JDBC installation is correct.

If you don’t receive feedback telling you that your installation is correct, you need to checkyour configuration Common problems include incorrectly set environment variables (PATH

and CLASSPATH) and mismatched versions of database connection drivers If you change the

environment variable values, you need to shut down and restart the command windows for the

changes to take effect

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Using the JDBC Classes

JDBC is implemented in Oracle via a set of Oracle-provided classes whose names begin with

oracle.sql, whereas the standard JDBC classes provided with the JDK begin with java.sql The

JdbcCheckup.java class, shown in the following listing, provides a good roadmap for beginning

JDBC programmers The JdbcCheckup.java class assumes you are using Oracle Net to connect

to the database

NOTE

The following code is provided by Oracle Your programmingstandards may implement these commands differently, such as bycalling System.out.println in place of System.out.print, or by placingthe brackets in different places

/*

* This sample can be used to check the JDBC installation.

* Just run it and provide the connect information It will select

* "Hello World" from the database.

public static void main (String args [])

throws SQLException, IOException {

// Prompt the user for connect information System.out.println ("Please enter information to test connection to the database");

String user;

String password;

String database;

user = readEntry ("user: ");

int slash_index = user.indexOf ('/');

if (slash_index != -1) {

password = user.substring (slash_index + 1);

user = user.substring (0, slash_index);

}

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The class then creates a statement, executes a hard-coded query, and prints the output:

// Create a statement Statement stmt = conn.createStatement ();

// Do the SQL "Hello World" thing ResultSet rset = stmt.executeQuery ("select 'Hello World' from dual");

while (rset.next ()) System.out.println (rset.getString (1));

The result set, statement, and connection are then closed and the program completes Note thatthere are two steps in executing the statement: First, it’s created via the call to the createStatement

method Second, it’s executed via the executeQuery method In place of executeQuery, you can

use execute or, if you’re running an insert, update, or delete SQL statement, executeUpdate If

you select a column value (instead of just a text string), you should specify that column value in

the print command, as shown in the following listing:

ResultSet rset =stmt.executeQuery("select User from dual");

while (rset.next ()) {

System.out.println (rset.getString ("USER"));

Using JDBC for Data Manipulation

Let’s combine the pieces that have been described so far—the basic connection syntax from this

chapter with the Java classes from Chapter 37 The example in this section will query the RADIUS_

VALS table, calculate an area for each value, and insert those values into the AREAS table Therefore,

it requires the use of the Java equivalent of a Cursor FOR loop, along with support for executable

commands and inserts.

For the example, put three records into the RADIUS_VALS table and delete any rows in theAREAS table:

delete from RADIUS_VALS;

insert into RADIUS_VALS(Radius) values (3);

delete from AREAS;

commit;

The following listing, JdbcCircle.java, contains the connection components from JdbcCheckup.java The executable commands for the circle area start in the section called RetrieveRadius, shown

in bold in the listing

// You need to import the java.sql package to use JDBC

import java.sql.*;

// We import java.io to be able to read from the command line

import java.io.*;

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import oracle.jdbc.pool.OracleDataSource;

class JdbcCircle {

public static void main (String args [])

throws SQLException, IOException {

// Prompt the user for connect information System.out.println ("Please enter information for connection");

String user;

String password;

String database;

user = readEntry ("user: ");

int slash_index = user.indexOf ('/');

if (slash_index != -1) {

password = user.substring (slash_index + 1);

user = user.substring (0, slash_index);

} else { password = readEntry ("password: ");

} database = readEntry ("database (a TNSNAME entry): ");

System.out.println ("Connecting to the database ");

// Create an OracleDataSource and set URL OracleDataSource ods = new OracleDataSource();

ods.setURL("jdbc:oracle:oci8:" + user + "/" + password + "@" + database);

// RetrieveRadius

ResultSet rset =stmt.executeQuery("select Radius from RADIUS_VALS");

while (rset.next ()) { // if you wish to print the values:

// System.out.println (rset.getInt ("RADIUS"));

int radInput = rset.getInt ("RADIUS");

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// Retrieve Radius value, calculate area:

double result = area(radInput);

// insert the value into AREAS String sql = "insert into AREAS values ("+radInput+","+result+ ")";

// If you want to print the SQL statement:

// System.out.println(sql);

// Create a statement for the insert:

Statement insArea = conn.createStatement();

// Execute the insert:

boolean insertResult = insArea.execute(sql);

// Utility function to calculate the area

public static double area (int rad) {

double pi=3.1415927;

double areacircle=pi*rad*rad;

return areacircle;

}

// Utility function to read a line from standard input

static String readEntry (String prompt) {

try { StringBuffer buffer = new StringBuffer ();

} catch (IOException e) { return "";

} }

}

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In the RetrieveRadius section, the query to be executed is passed to the executeQuery method.

The result of the query is stored in a variable namedrset

// RetrieveRadius

ResultSet rset =stmt.executeQuery("select Radius from RADIUS_VALS");

As noted in Chapter 37, you can use a WHILE loop in Java to approximate the functionality of aPL/SQL Cursor FOR loop The following part of the RetrieveRadius section uses the getInt method

to retrieve the integer Radius value from the result set If the value had been a string, the getString

method would have been used instead

while (rset.next ()) { // if you wish to print the values:

// System.out.println (rset.getInt ("RADIUS"));

int radInput = rset.getInt ("RADIUS");

TheradInput variable, assigned in the preceding listing, can now be used as input to the areacalculation The area calculation is performed via a utility function (see the full JdbcCircle listing)

based on the code examples presented in Chapter 37

// Retrieve Radius value, calculate area:

double result = area(radInput);

We now have the radius value (theradInput variable) and the area value (the result variable)

to use when populating the AREAS table Let’s build the insert statement to perform the insert First,

use the string concatenation functions in Java to concatenate text with the variable values Make

sure not to include a semicolon at the end of the generated command string

// insert the value into AREAS String sql = "insert into AREAS values ("+radInput+","+result+ ")";

// If you want to print the SQL statement:

// System.out.println(sql);

Next, create a statement and execute the insert command (via a call to thesql variable thatcontains the command text):

// Create a statement for the insert:

Statement insArea = conn.createStatement();

// Execute the insert:

boolean insertResult = insArea.execute(sql);

Did it work? Compile and execute the JdbcCircle.java file:

javac JdbcCircle.java

java JdbcCircle

When prompted, provide a username and password for the owner of the RADIUS_VALS andAREAS tables After the program completes, check the AREAS table:

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select * from AREAS

order by Radius;

RADIUS AREA - -

FOR loop, assigning variables, generating a dynamic SQL statement, and executing DML

JDBC supports extensive programming features, including statement caching, connectioncaching, connection failover, distributed transactions, support for LOBs, and more See theOracle

Database JDBC Developer’s Guide and Reference for further details on the use of JDBC

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39

Java Stored Procedures

Tiêu đề	Object-Relational Databases and Oracle Database 10g
Trường học	Unknown University
Chuyên ngành	Database Technologies
Thể loại	Textbook chapter
Năm xuất bản	2004
Thành phố	Unknown City

Định dạng
Số trang	135
Dung lượng	459,62 KB