	CS 683 Emerging Technologies Spring Semester, 2003 AspectJ Syntax 2
	Previous Lecture Notes Index Next © 2003, All Rights Reserved, SDSU & Roger Whitney San Diego State University -- This page last updated 30-Jan-03

Contents of Doc 4, AspectJ Syntax 2

Reference

Reading
The AspectJ Programming Guide

Section 1 Getting Started with AspectJ
Section 2 The AspectJ Language


Doc 4, AspectJ Syntax 2 Slide # 2

AspectJ Syntax 2

Viewing Aspect Source Code
ajc -preprocess @Hello.lst

Will write out the Java source generated to implement the aspect

public class Hello {
   public Hello() {
      this(5);
      System.out.println("Hello()");   
   }
   
   public Hello(int x) {
      System.out.println("Hello(" + x + ")");
   }
   
   public static void main(String[] args ) {
      new Hello();
   }
}

public aspect HelloAspect {
   before() : call( Hello.new(..) ) {
      System.out.println( "call-before");
   }
   
   before() : execution(Hello.new(..)) {
      System.out.println( "execution-before");
   }
}
Doc 4, AspectJ Syntax 2 Slide # 3

Generated Hello.java

/*   Generated by AspectJ version 1.0.6 */
public class Hello {
  public Hello() {
    this(5);
    HelloAspect.aspectInstance.before1$ajc();
    ;
    System.out.println("Hello()");
  } 
  public Hello(int x) {
    super();
    HelloAspect.aspectInstance.before1$ajc();
    System.out.println("Hello(" + x + ")");
  } 
  public static void main(String[] args) {
    Hello.new$constructor_call();
  } 
  private static Hello new$constructor_call() {
    HelloAspect.aspectInstance.before0$ajc();
    return new Hello();
  } 
   
}


Doc 4, AspectJ Syntax 2 Slide # 4

Generated HelloAspect

/*   Generated by AspectJ version 1.0.6 */
public class HelloAspect {
  public final void before0$ajc() {
    System.out.println("call-before");
  } 
  public final void before1$ajc() {
    System.out.println("execution-before");
  } 
  public HelloAspect() {
    super();
  } 
  public static HelloAspect aspectInstance;
  public static HelloAspect aspectOf() {
    return HelloAspect.aspectInstance;
  } 
  public static boolean hasAspect() {
    return HelloAspect.aspectInstance != null;
  } 
  static {
    HelloAspect.aspectInstance = new HelloAspect();
  } 
}


Doc 4, AspectJ Syntax 2 Slide # 5

cflow

cflow(Pointcut)

Picks out join points in the Pointcut and code called by the Pointcut

cflowbelow(Pointcut)

Picks out join points in the code called by the Pointcut but not directly in the Pointcut


Doc 4, AspectJ Syntax 2 Slide # 6

Example

public class Hello {
   public void c() {
      System.out.println("C");
   }
   
   public void b() {
      System.out.println("B");
      c();
   }
   
   public void a() {
      System.out.println( "A");
      b();
   }
      
   public static void main(String[] args ) {
    Hello test = new Hello();
   test.c();
   test.b();
   test.a();
   }
}


Doc 4, AspectJ Syntax 2 Slide # 7

Same behavior when referring to b()

public aspect HelloAspect {
   before() : call( * b() ) && withincode( * a() ) {
      System.out.println( "call-within");
   }
      
  before() : call( * b() ) && cflow( call(* a()) ) {
      System.out.println( "call-cflow");
   }
}

Output
C
B
C
A
call-within
call-cflow
B
C


Doc 4, AspectJ Syntax 2 Slide # 8

Different behavior when referring to c()

public aspect HelloAspect {
   before() : call( * c() ) && withincode( * a() ) {
      System.out.println( "call-within");
   }
      
  before() : call( * c() ) && cflow( call(* a()) ) {
      System.out.println( "call-cflow");
   }
}

Output
C
B
C
A
B
call-cflow
C


Doc 4, AspectJ Syntax 2 Slide # 9

Recursive calls

An example to show how to pick off the start and tail of recursion

public class Hello {
   
   public void a(int repeats) {
      System.out.println( "A" + repeats);
      if (repeats > 0)
         a( repeats - 1);
   }
      
   public static void main(String[] args ) {
    Hello test = new Hello();
   test.a(4);
   }
} 
Doc 4, AspectJ Syntax 2 Slide # 10

The Aspect

public aspect HelloAspect {
   before() : call( * a(int) ) && withincode( * a(int) ) {
      System.out.println( "call-within");
   }
      
  before() : call( * a(int) ) && !cflowbelow( call(* a(int)) ) {
      System.out.println( "call-cflow");
   }
}

Output call-cflow
A4
call-within
A3
call-within
A2
call-within
A1
call-within
A0


Doc 4, AspectJ Syntax 2 Slide # 11

Some More on Matching

Boxing

public class Hello {
   public void a(int repeats) {
      System.out.println( "A" + repeats);
   }
   
   public static void main(String[] args ) {
    Hello test = new Hello();
   test.a(4);
   }
}

public aspect HelloAspect {
   before(Object boxed) : call( * a(int) ) && args(boxed) {
      System.out.println( "Match " + boxed);
   }
}

Output
Match 4
A4


Doc 4, AspectJ Syntax 2 Slide # 12

Wildcards Again

* matches zero or more characters except for “.”

.. matches any sequence of characters that start & end with a ‘.’

target(java.util.*)

picks out all types in the java.util package but not inner types
Does not pick out java.util.Map.Entry

target(java.util..)

Still does not pick out java.util.Map.Entry

target(java.util..*)

Picks out all types in java.util package
Picks out all inner types like java.util. Map.Entry
Picks out java.util.logging.Handler


Doc 4, AspectJ Syntax 2 Slide # 13

Subtype Patterns

Matches all subtypes
Comes after a type name pattern

call( Hello+.new() )
call (( Hello+ && ! Hello).new() )

public class Hello {
   public Hello() {
      System.out.println("Hello Contructor" );
   }
   }

public class Child extends Hello {
   public Child() {
      System.out.println("Child Contructor" );
   }
}


Doc 4, AspectJ Syntax 2 Slide # 14

Examples

public aspect HelloAspect {
   before() : call( Hello.new() ) {
      System.out.println( "Match ");
   }
}

new Hello() Produces Match
Hello Constructor

new Child() Produces Hello Constructor
Child Constructor

public aspect HelloAspect {
   before() : call( Hello+.new() ) {
      System.out.println( "Match ");
   }
}

new Hello() Produces Match
Hello Constructor

new Child() Produces Match
Hello Constructor
Child Constructor


Doc 4, AspectJ Syntax 2 Slide # 15

public aspect HelloAspect {
   before() : call( (Hello+ && ! Hello).new() ) {
      System.out.println( "Match ");
   }
}

new Hello() Produces Hello Constructor

new Child() Produces Match
Hello Constructor
Child Constructor


Doc 4, AspectJ Syntax 2 Slide # 16

After Returning
after() returning()

Run after a join point returns normally

public class Hello {
   int increase(int value) {
      return value + 1;
   }
   
   public static void main(String[] args ) {
    Hello test = new Hello();
   test.increase(5);
   }
}

public aspect HelloAspect {
   after() returning() : call( int increase(int) ) {
      System.out.println( "No parameters ");
   }
   after() returning(Object returnBoxed) : call( int increase(int) ) {
      System.out.println( "Returned " + returnBoxed);
   }
 
   after(int arg) returning(int returned) : call( int increase(int) ) && args(arg) {
      System.out.println( "In " + arg + " Out " + returned);
   }
}

Output No parameters
Returned 6
In 5 Out 6


Doc 4, AspectJ Syntax 2 Slide # 17

Exceptions

Catching & After

import java.io.IOException;
   
public class Hello {
   
   int a(int value) throws IOException {
      if (value == 3) 
         throw new IOException();
      System.out.println( "In a");
      return value + 1;
   }
      
   public static void main(String[] args ) {
      Hello test = new Hello();
      try {
         test.a( Integer.parseInt(args[0]) );
      } 
      catch (IOException example) {
         System.out.println( "In Handler"); 
      }
   }
}


Doc 4, AspectJ Syntax 2 Slide # 18

The Aspects

import java.io.IOException;

public aspect HelloAspect {
   after()  : call( int a(int) ) {
      System.out.println( "After ");
   }
   after()  returning : call( int a(int) ) {
      System.out.println( "returning ");
   }
   after()  throwing : call( int a(int) ) {
      System.out.println( "throwing ");
   }
   after() :handler(IOException) {
   System.out.println("After handler");
    }
}

Running java Hello 4 In a
After
Returning

Running java Hello 3 After
throwing
In Handler
After handler


Doc 4, AspectJ Syntax 2 Slide # 19

Warning about not importing Exception
The following aspect does compile and will run, but not the way one thinks

public aspect HelloAspect {
   after()  : call( int a(int) ) {
      System.out.println( "After ");
   }
   after()  returning : call( int a(int) ) {
      System.out.println( "returning ");
   }
   after()  throwing : call( int a(int) ) {
      System.out.println( "throwing ");
   }
   after() :handler(IOException) {
   System.out.println("After handler");
    }
}

Running java Hello 3 After
throwing
In Handler


Doc 4, AspectJ Syntax 2 Slide # 20

Throwing Checked Exceptions in Advice

Advice must explicitly declare if it throws an exception

import java.io.IOException;
   
public aspect HelloAspect {
   before() throws IOException : call( int a(int)) {
      throw new IOException();
   }
   after() throws IOException : call( int a(int)) {
      throw new IOException();
   }
   
   after() throwing(IOException e) throws IOException  : call( int a(int)) {
      throw new IOException();
   }
}


Doc 4, AspectJ Syntax 2 Slide # 21

Restrictions on Throwing Exceptions
Exceptions a join point in AspectJ may throw are:

method call and execution

Checked exceptions declared by the target method's throwsclause

constructor call and execution

Checked exceptions declared by the target constructor's throwsclause

field get and set

No checked exceptions can be thrown from these join points

exception handler execution

Exceptions that can be thrown by the target exception handler

static initializer execution

no checked exceptions can be thrown from these join points

pre-initialization, and initialization

Any exception that is in the throws clause of all constructors of the initialized class


Doc 4, AspectJ Syntax 2 Slide # 22

Illegal Example
This advice will not compile

public class Hello {
    void b() {
      System.out.println("In b");
   }
}

import java.io.IOException;
   
public aspect HelloAspect {
   before() throws IOException : call( void b()) {
      throw new IOException();
   }
}


Doc 4, AspectJ Syntax 2 Slide # 23

Throwing Uncheck Exceptions in Advice

Unchecked exceptions can be thrown in advice

public class Hello {
    void b() {
      System.out.println("In b");
   }
   public static void main(String[] args ) {
      Hello test = new Hello();
      test.b();
   }
}

public aspect HelloAspect {
   before()  : call( void b()) {
      throw new ArithmeticException ();
   }
}


Doc 4, AspectJ Syntax 2 Slide # 24

Warnings & Errors

Can specify that a join point should not be reached

Compiler will signal the problem

Forms:

declare error : Pointcut : String

declare warning: Pointcut : String

Example

public class Hello {
   void b() {
      System.out.println("In b");
   }
   public static void main(String[] args ) {
      Hello test = new Hello();
      test.b();
   }
}
public aspect HelloAspect {
   declare error  : call( void b()) : "Error string displayed by compiler";   
} 
Doc 4, AspectJ Syntax 2 Slide # 25

Aspect Extension

An aspect may

Extend an abstract aspect
Extend a class
Implement an interface

A class cannot extend an aspect

Aspect Extending an Aspect Example

public abstract  aspect ParentAspect {
   pointcut methodB() : call(* b(..) );
} 
   
aspect ChildAspect extends ParentAspect {
   before() : methodB() {
      System.out.println(" Advise " );
   }
}
Doc 4, AspectJ Syntax 2 Slide # 26

Aspect Extending a Class

public class Hello {
    void b() {
      System.out.println("In b");
   }
}

public  aspect HelloAspect extends Hello {
   before() : call(* foo() ) {
      b();
   }
}


Doc 4, AspectJ Syntax 2 Slide # 27

Aspect State & Methods

Aspects can have state and methods

Aspects normally are created as singletons

AspectName.aspectOf() returns the singleton

public  aspect HelloAspect {
   
   private int callCount = 0;
   
   public int getCount() {
      return callCount;
   }
   
   public static void main(String[] arguments ) {
      HelloAspect aspect = HelloAspect.aspectOf();
      System.out.println( "Count is " + aspect.getCount() );
   }
   
   before() : call(* *(..) ) {
      callCount++;
   }
}

Result of running java HelloAspect Count is 3

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