javassist

Class CodeConverter

java.lang.Object

javassist.CodeConverter

public class CodeConverter
extends 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:

CtClass.instrument(CodeConverter), CtBehavior.instrument(CodeConverter), ExprEditor

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static interface	CodeConverter.ArrayAccessReplacementMethodNames Interface containing the method names to be used as array access replacements.
static class	CodeConverter.DefaultArrayAccessReplacementMethodNames Default implementation of the ArrayAccessReplacementMethodNames interface giving default values for method names to be used for replacing accesses to array elements.

Field Summary

Fields

Modifier and Type	Field and Description
protected Transformer	transformers

Constructor Summary

Constructors

Constructor and Description
CodeConverter()

Method Summary

Methods

Modifier and Type	Method and Description
protected void	doit(CtClass clazz, MethodInfo minfo, ConstPool cp) Performs code conversion.
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, CtClass newClass, 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 will be invoked.
void	redirectMethodCall(String oldMethodName, CtMethod newMethod) Correct invocations to a method that has been renamed.
void	replaceArrayAccess(CtClass calledClass, CodeConverter.ArrayAccessReplacementMethodNames names) Modify a method body, so that ALL accesses to an array are replaced with calls to static methods within another class.
void	replaceFieldRead(CtField field, CtClass calledClass, 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, CtClass calledClass, 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(CtClass oldClass, CtClass newClass) Modify a method body so that instantiation of the class specified by oldClass is replaced with instantiation of another class newClass.
void	replaceNew(CtClass newClass, CtClass calledClass, 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

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

Field Detail

transformers

protected Transformer transformers

Constructor Detail

CodeConverter

public CodeConverter()

Method Detail

replaceNew

public void replaceNew(CtClass newClass,
              CtClass calledClass,
              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.

replaceNew

public void replaceNew(CtClass oldClass,
              CtClass newClass)

Modify a method body so that instantiation of the class specified by oldClass is replaced with instantiation of another class newClass. For example, replaceNew(ctPoint, ctPoint2) (where ctPoint and ctPoint2 are compile-time classes for class Point and class Point2, respectively) replaces all occurrences of:

new Point(x, y)

in the method body with:

new Point2(x, y)

Note that Point2 must be type-compatible with Point. It must have the same set of methods, fields, and constructors as the replaced class.

redirectFieldAccess

public void redirectFieldAccess(CtField field,
                       CtClass newClass,
                       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,
                    CtClass calledClass,
                    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,
                     CtClass calledClass,
                     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.

replaceArrayAccess

public void replaceArrayAccess(CtClass calledClass,
                      CodeConverter.ArrayAccessReplacementMethodNames names)
                        throws NotFoundException

Modify a method body, so that ALL accesses to an array are replaced with calls to static methods within another class. In the case of reading an element from the array, this is replaced with a call to a static method with the array and the index as arguments, the return value is the value read from the array. If writing to an array, this is replaced with a call to a static method with the array, index and new value as parameters, the return value of the static method is void.

The calledClass parameter is the class containing the static methods to be used for array replacement. The names parameter points to an implementation of ArrayAccessReplacementMethodNames which specifies the names of the method to be used for access for each type of array. For example reading from an int[] will require a different method than if writing to an int[], and writing to a long[] will require a different method than if writing to a byte[]. If the implementation of ArrayAccessReplacementMethodNames does not contain the name for access for a type of array, that access is not replaced.

A default implementation of ArrayAccessReplacementMethodNames called DefaultArrayAccessReplacementMethodNames has been provided and is what is used in the following example. This also assumes that 'foo.ArrayAdvisor' is the name of the CtClass passed in.

If we have the following class:

class POJO{
    int[] ints = new int[]{1, 2, 3, 4, 5};
    long[] longs = new int[]{10, 20, 30};
    Object objects = new Object[]{true, false};
    Integer[] integers = new Integer[]{new Integer(10)};
 }
 

and this is accessed as:

POJO p = new POJO();
 
 //Write to int array
 p.ints[2] = 7;
 
 //Read from int array
 int i = p.ints[2];
 
 //Write to long array
 p.longs[2] = 1000L;
 
 //Read from long array
 long l = p.longs[2];
 
 //Write to Object array
 p.objects[2] = "Hello";
 
 //Read from Object array
 Object o = p.objects[2];
 
 //Write to Integer array
 Integer integer = new Integer(5);
 p.integers[0] = integer;
 
 //Read from Object array
 integer = p.integers[0];
 

Following instrumentation we will have

POJO p = new POJO();
 
 //Write to int array
 ArrayAdvisor.arrayWriteInt(p.ints, 2, 7);
 
 //Read from int array
 int i = ArrayAdvisor.arrayReadInt(p.ints, 2);
 
 //Write to long array
 ArrayAdvisor.arrayWriteLong(p.longs, 2, 1000L);
 
 //Read from long array
 long l = ArrayAdvisor.arrayReadLong(p.longs, 2);
 
 //Write to Object array
 ArrayAdvisor.arrayWriteObject(p.objects, 2, "Hello");
 
 //Read from Object array
 Object o = ArrayAdvisor.arrayReadObject(p.objects, 2);
 
 //Write to Integer array
 Integer integer = new Integer(5);
 ArrayAdvisor.arrayWriteObject(p.integers, 0, integer);
 
 //Read from Object array
 integer = ArrayAdvisor.arrayWriteObject(p.integers, 0);
 

Parameters:

calledClass - the class containing the static methods.

names - contains the names of the methods to replace the different kinds of array access with.

Throws:

NotFoundException

See Also:

CodeConverter.DefaultArrayAccessReplacementMethodNames

redirectMethodCall

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

Modify method invocations in a method body so that a different method will be 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

redirectMethodCall

public void redirectMethodCall(String oldMethodName,
                      CtMethod newMethod)
                        throws CannotCompileException

Correct invocations to a method that has been renamed. If a method is renamed, calls to that method must be also modified so that the method with the new name will be called.

The method must be declared in the same class before and after it is renamed.

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.

Parameters:

oldMethodName - the old name of the method.

newMethod - the method with the new name.

Throws:

CannotCompileException

See Also:

CtMethod.setName(String)

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

doit

protected void doit(CtClass clazz,
        MethodInfo minfo,
        ConstPool cp)
             throws CannotCompileException

Performs code conversion.

Throws:

CannotCompileException