classUtils.javassist
Class CodeConverter

java.lang.Object
  classUtils.javassist.CodeConverter

public class CodeConverter
extends java.lang.Object

Simple translator of method bodies (also see the javassist.expr package).

Instances of this class specifies how to instrument of the bytecodes representing a method body. They are passed to CtClass.instrument() or CtMethod.instrument() as a parameter.

Example:

 ClassPool cp = ClassPool.getDefault();
 CtClass point = cp.get("Point");
 CtClass singleton = cp.get("Singleton");
 CtClass client = cp.get("Client");
 CodeConverter conv = new CodeConverter();
 conv.replaceNew(point, singleton, "makePoint");
 client.instrument(conv);
 

This program substitutes "Singleton.makePoint()" for all occurrences of "new Point()" appearing in methods declared in a Client class.

See Also:

CompileTimeClass.instrument(CodeConverter), CtMethod.instrument(CodeConverter), ExprEditor

Constructor Summary

CodeConverter()

Method Summary

void	insertAfterMethod(CtMethod origMethod, CtMethod afterMethod) Inserts a call to another method after an existing method call.
void	insertBeforeMethod(CtMethod origMethod, CtMethod beforeMethod) Insert a call to another method before an existing method call.
void	redirectFieldAccess(CtField field, CompileTimeClass newClass, java.lang.String newFieldname) Modify a method body so that field read/write expressions access a different field from the original one.
void	redirectMethodCall(CtMethod origMethod, CtMethod substMethod) Modify method invocations in a method body so that a different method is invoked.
void	replaceFieldRead(CtField field, CompileTimeClass calledClass, java.lang.String calledMethod) Modify a method body so that an expression reading the specified field is replaced with a call to the specified static method.
void	replaceFieldWrite(CtField field, CompileTimeClass calledClass, java.lang.String calledMethod) Modify a method body so that an expression writing the specified field is replaced with a call to the specified static method.
void	replaceNew(CompileTimeClass newClass, CompileTimeClass calledClass, java.lang.String calledMethod) Modify a method body so that instantiation of the specified class is replaced with a call to the specified static method.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

CodeConverter

public CodeConverter()

Method Detail

replaceNew

public void replaceNew(CompileTimeClass newClass,
                       CompileTimeClass calledClass,
                       java.lang.String calledMethod)

Modify a method body so that instantiation of the specified class is replaced with a call to the specified static method. For example, replaceNew(ctPoint, ctSingleton, "createPoint") (where ctPoint and ctSingleton are compile-time classes for class Point and class Singleton, respectively) replaces all occurrences of:

new Point(x, y)

in the method body with:

Singleton.createPoint(x, y)

This enables to intercept instantiation of Point and change the samentics. For example, the following createPoint() implements the singleton pattern:

public static Point createPoint(int x, int y) {
     if (aPoint == null)
         aPoint = new Point(x, y);
     return aPoint;
 }
 

The static method call substituted for the original new expression must be able to receive the same set of parameters as the original constructor. If there are multiple constructors with different parameter types, then there must be multiple static methods with the same name but different parameter types.

The return type of the substituted static method must be the exactly same as the type of the instantiated class specified by newClass.

Parameters:

newClass - the instantiated class.

calledClass - the class in which the static method is declared.

calledMethod - the name of the static method.

redirectFieldAccess

public void redirectFieldAccess(CtField field,
                                CompileTimeClass newClass,
                                java.lang.String newFieldname)

Modify a method body so that field read/write expressions access a different field from the original one.

Note that this method changes only the filed name and the class declaring the field; the type of the target object does not change. Therefore, the substituted field must be declared in the same class or a superclass of the original class.

Also, clazz and newClass must specify the class directly declaring the field. They must not specify a subclass of that class.

Parameters:

field - the originally accessed field.

newClass - the class declaring the substituted field.

newFieldname - the name of the substituted field.

replaceFieldRead

public void replaceFieldRead(CtField field,
                             CompileTimeClass calledClass,
                             java.lang.String calledMethod)

Modify a method body so that an expression reading the specified field is replaced with a call to the specified static method. This static method receives the target object of the original read expression as a parameter. It must return a value of the same type as the field.

For example, the program below

Point p = new Point();
 int newX = p.x + 3;

can be translated into:

Point p = new Point();
 int newX = Accessor.readX(p) + 3;

where

public class Accessor {
     public static int readX(Object target) { ... }
 }

The type of the parameter of readX() must be java.lang.Object independently of the actual type of target. The return type must be the same as the field type.

Parameters:

field - the field.

calledClass - the class in which the static method is declared.

calledMethod - the name of the static method.

replaceFieldWrite

public void replaceFieldWrite(CtField field,
                              CompileTimeClass calledClass,
                              java.lang.String calledMethod)

Modify a method body so that an expression writing the specified field is replaced with a call to the specified static method. This static method receives two parameters: the target object of the original write expression and the assigned value. The return type of the static method is void.

For example, the program below

Point p = new Point();
 p.x = 3;

can be translated into:

Point p = new Point();
 Accessor.writeX(3);

where

public class Accessor {
     public static void writeX(Object target, int value) { ... }
 }

The type of the first parameter of writeX() must be java.lang.Object independently of the actual type of target. The type of the second parameter is the same as the field type.

Parameters:

field - the field.

calledClass - the class in which the static method is declared.

calledMethod - the name of the static method.

redirectMethodCall

public void redirectMethodCall(CtMethod origMethod,
                               CtMethod substMethod)
                        throws CannotCompileException

Modify method invocations in a method body so that a different method is invoked.

Note that the target object, the parameters, or the type of invocation (static method call, interface call, or private method call) are not modified. Only the method name is changed. The substituted method must have the same signature that the original one has. If the original method is a static method, the substituted method must be static.

Parameters:

origMethod - original method

substMethod - substituted method

Throws:

CannotCompileException

insertBeforeMethod

public void insertBeforeMethod(CtMethod origMethod,
                               CtMethod beforeMethod)
                        throws CannotCompileException

Insert a call to another method before an existing method call. That "before" method must be static. The return type must be void. As parameters, the before method receives the target object and all the parameters to the originally invoked method. For example, if the originally invoked method is move():

class Point {
     Point move(int x, int y) { ... }
 }

Then the before method must be something like this:

class Verbose {
     static void print(Point target, int x, int y) { ... }
 }

The CodeConverter would translate bytecode equivalent to:

Point p2 = p.move(x + y, 0);

into the bytecode equivalent to:

int tmp1 = x + y;
 int tmp2 = 0;
 Verbose.print(p, tmp1, tmp2);
 Point p2 = p.move(tmp1, tmp2);

Parameters:

origMethod - the method originally invoked.

beforeMethod - the method invoked before origMethod.

Throws:

CannotCompileException

insertAfterMethod

public void insertAfterMethod(CtMethod origMethod,
                              CtMethod afterMethod)
                       throws CannotCompileException

Inserts a call to another method after an existing method call. That "after" method must be static. The return type must be void. As parameters, the after method receives the target object and all the parameters to the originally invoked method. For example, if the originally invoked method is move():

class Point {
     Point move(int x, int y) { ... }
 }

Then the after method must be something like this:

class Verbose {
     static void print(Point target, int x, int y) { ... }
 }

The CodeConverter would translate bytecode equivalent to:

Point p2 = p.move(x + y, 0);

into the bytecode equivalent to:

int tmp1 = x + y;
 int tmp2 = 0;
 Point p2 = p.move(tmp1, tmp2);
 Verbose.print(p, tmp1, tmp2);

Parameters:

origMethod - the method originally invoked.

afterMethod - the method invoked after origMethod.

Throws:

CannotCompileException