http://www.javassist.org/: Javassist (JAVA programming ASSISTant) makes Java bytecode manipulation simple. It is a class library for editing bytecodes in Java. (Shigeru Chiba, www.javassist.org) 
/*
 * Javassist, a Java-bytecode translator toolkit.
 * Copyright (C) 1999- Shigeru Chiba. All Rights Reserved.
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License.  Alternatively, the contents of this file may be used under
 * the terms of the GNU Lesser General Public License Version 2.1 or later,
 * or the Apache License Version 2.0.
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 */

package javassist;

import javassist.bytecode.BadBytecode;
import javassist.bytecode.Bytecode;
import javassist.bytecode.ClassFile;
import javassist.bytecode.CodeAttribute;
import javassist.bytecode.CodeIterator;
import javassist.bytecode.ConstPool;
import javassist.bytecode.Descriptor;
import javassist.bytecode.MethodInfo;
import javassist.bytecode.Opcode;
import javassist.compiler.CompileError;
import javassist.compiler.Javac;


An instance of CtConstructor represents a constructor.
It may represent a static constructor
(class initializer).  To distinguish a constructor and a class
initializer, call isClassInitializer().

See the super class CtBehavior as well since
a number of useful methods are in CtBehavior.

See Also:
/**
 * An instance of CtConstructor represents a constructor.
 * It may represent a static constructor
 * (class initializer).  To distinguish a constructor and a class
 * initializer, call <code>isClassInitializer()</code>.
 *
 * <p>See the super class <code>CtBehavior</code> as well since
 * a number of useful methods are in <code>CtBehavior</code>.
 *
 * @see CtClass#getDeclaredConstructors()
 * @see CtClass#getClassInitializer()
 * @see CtNewConstructor
 */
public final class CtConstructor extends CtBehavior {
    protected CtConstructor(MethodInfo minfo, CtClass declaring) {
        super(declaring, minfo);
    }

    
Creates a constructor with no constructor body.
The created constructor
must be added to a class with CtClass.addConstructor().

The created constructor does not include a constructor body,
which must be specified with setBody().

Params:
  • declaring –         the class to which the created method is added.
  • parameters –        a list of the parameter types
See Also:
/**
     * Creates a constructor with no constructor body.
     * The created constructor
     * must be added to a class with <code>CtClass.addConstructor()</code>.
     *
     * <p>The created constructor does not include a constructor body,
     * which must be specified with <code>setBody()</code>.
     *
     * @param declaring         the class to which the created method is added.
     * @param parameters        a list of the parameter types
     *
     * @see CtClass#addConstructor(CtConstructor)
     * @see CtConstructor#setBody(String)
     * @see CtConstructor#setBody(CtConstructor,ClassMap)
     */
    public CtConstructor(CtClass[] parameters, CtClass declaring) {
        this((MethodInfo)null, declaring);
        ConstPool cp = declaring.getClassFile2().getConstPool();
        String desc = Descriptor.ofConstructor(parameters);
        methodInfo = new MethodInfo(cp, "<init>", desc);
        setModifiers(Modifier.PUBLIC);
    }

    
Creates a copy of a CtConstructor object.
The created constructor must be
added to a class with CtClass.addConstructor().

All occurrences of class names in the created constructor
are replaced with names specified by
map if map is not null.

By default, all the occurrences of the names of the class
declaring src and the superclass are replaced
with the name of the class and the superclass that
the created constructor is added to.
This is done whichever map is null or not.
To prevent this replacement, call ClassMap.fix()
or put() to explicitly specify replacement.

Note: if the .class notation (for example,
String.class) is included in an expression, the
Javac compiler may produce a helper method.
Since this constructor never
copies this helper method, the programmers have the responsiblity of
copying it.  Otherwise, use Class.forName() in the
expression.

Params:
  • src –       the source method.
  • declaring –    the class to which the created method is added.
  • map –       the hashtable associating original class names
                 with substituted names.
                 It can be null.
See Also:
/**
     * Creates a copy of a <code>CtConstructor</code> object.
     * The created constructor must be
     * added to a class with <code>CtClass.addConstructor()</code>.
     *
     * <p>All occurrences of class names in the created constructor
     * are replaced with names specified by
     * <code>map</code> if <code>map</code> is not <code>null</code>.
     *
     * <p>By default, all the occurrences of the names of the class
     * declaring <code>src</code> and the superclass are replaced
     * with the name of the class and the superclass that
     * the created constructor is added to.
     * This is done whichever <code>map</code> is null or not.
     * To prevent this replacement, call <code>ClassMap.fix()</code>
     * or <code>put()</code> to explicitly specify replacement.
     *
     * <p><b>Note:</b> if the <code>.class</code> notation (for example,
     * <code>String.class</code>) is included in an expression, the
     * Javac compiler may produce a helper method.
     * Since this constructor never
     * copies this helper method, the programmers have the responsiblity of
     * copying it.  Otherwise, use <code>Class.forName()</code> in the
     * expression.
     *
     * @param src       the source method.
     * @param declaring    the class to which the created method is added.
     * @param map       the hashtable associating original class names
     *                  with substituted names.
     *                  It can be <code>null</code>.
     *
     * @see CtClass#addConstructor(CtConstructor)
     * @see ClassMap#fix(String)
     */
    public CtConstructor(CtConstructor src, CtClass declaring, ClassMap map)
        throws CannotCompileException
    {
        this((MethodInfo)null, declaring);
        copy(src, true, map);
    }

    
Returns true if this object represents a constructor.

/**
     * Returns true if this object represents a constructor.
     */
    public boolean isConstructor() {
        return methodInfo.isConstructor();
    }

    
Returns true if this object represents a static initializer.

/**
     * Returns true if this object represents a static initializer.
     */
    public boolean isClassInitializer() {
        return methodInfo.isStaticInitializer();
    }

    
Returns the constructor name followed by parameter types
such as javassist.CtConstructor(CtClass[],CtClass).

Since:3.5
/**
     * Returns the constructor name followed by parameter types
     * such as <code>javassist.CtConstructor(CtClass[],CtClass)</code>.
     *
     * @since 3.5
     */
    @Override
    public String getLongName() {
        return getDeclaringClass().getName()
               + (isConstructor() ? Descriptor.toString(getSignature())
                                  : ("." + MethodInfo.nameClinit + "()"));
    }

    
Obtains the name of this constructor.
It is the same as the simple name of the class declaring this
constructor.  If this object represents a class initializer,
then this method returns "<clinit>".

/**
     * Obtains the name of this constructor.
     * It is the same as the simple name of the class declaring this
     * constructor.  If this object represents a class initializer,
     * then this method returns <code>"&lt;clinit&gt;"</code>.
     */
    @Override
    public String getName() {
        if (methodInfo.isStaticInitializer())
            return MethodInfo.nameClinit;
        return declaringClass.getSimpleName();
    }

    
Returns true if the constructor (or static initializer)
is the default one.  This method returns true if the constructor
takes some arguments but it does not perform anything except
calling super() (the no-argument constructor of
the super class).

/**
     * Returns true if the constructor (or static initializer)
     * is the default one.  This method returns true if the constructor
     * takes some arguments but it does not perform anything except
     * calling <code>super()</code> (the no-argument constructor of
     * the super class).
     */
    @Override
    public boolean isEmpty() {
        CodeAttribute ca = getMethodInfo2().getCodeAttribute();
        if (ca == null)
            return false;       // native or abstract??
                                // they are not allowed, though.

        ConstPool cp = ca.getConstPool();
        CodeIterator it = ca.iterator();
        try {
            int pos, desc;
            int op0 = it.byteAt(it.next());
            return op0 == Opcode.RETURN     // empty static initializer
                || (op0 == Opcode.ALOAD_0
                    && it.byteAt(pos = it.next()) == Opcode.INVOKESPECIAL
                    && (desc = cp.isConstructor(getSuperclassName(),
                                                it.u16bitAt(pos + 1))) != 0
                    && "()V".equals(cp.getUtf8Info(desc))
                    && it.byteAt(it.next()) == Opcode.RETURN
                    && !it.hasNext());
        }
        catch (BadBytecode e) {}
        return false;
    }

    private String getSuperclassName() {
        ClassFile cf = declaringClass.getClassFile2();
        return cf.getSuperclass();
    }

    
Returns true if this constructor calls a constructor
of the super class.  This method returns false if it
calls another constructor of this class by this(). 

/**
     * Returns true if this constructor calls a constructor
     * of the super class.  This method returns false if it
     * calls another constructor of this class by <code>this()</code>. 
     */
    public boolean callsSuper() throws CannotCompileException {
        CodeAttribute codeAttr = methodInfo.getCodeAttribute();
        if (codeAttr != null) {
            CodeIterator it = codeAttr.iterator();
            try {
                int index = it.skipSuperConstructor();
                return index >= 0;
            }
            catch (BadBytecode e) {
                throw new CannotCompileException(e);
            }
        }

        return false;
    }

    
Sets a constructor body.

Params:
  • src –       the source code representing the constructor body.
                 It must be a single statement or block.
                 If it is null, the substituted
                 constructor body does nothing except calling
                 super().
/**
     * Sets a constructor body.
     *
     * @param src       the source code representing the constructor body.
     *                  It must be a single statement or block.
     *                  If it is <code>null</code>, the substituted
     *                  constructor body does nothing except calling
     *                  <code>super()</code>.
     */
    @Override
    public void setBody(String src) throws CannotCompileException {
        if (src == null)
            if (isClassInitializer())
                src = ";";
            else
                src = "super();";

        super.setBody(src);
    }

    
Copies a constructor body from another constructor.

All occurrences of the class names in the copied body
are replaced with the names specified by
map if map is not null.

Params:
  • src –       the method that the body is copied from.
  • map –       the hashtable associating original class names
                 with substituted names.
                 It can be null.
/**
     * Copies a constructor body from another constructor.
     *
     * <p>All occurrences of the class names in the copied body
     * are replaced with the names specified by
     * <code>map</code> if <code>map</code> is not <code>null</code>.
     *
     * @param src       the method that the body is copied from.
     * @param map       the hashtable associating original class names
     *                  with substituted names.
     *                  It can be <code>null</code>.
     */
    public void setBody(CtConstructor src, ClassMap map)
        throws CannotCompileException
    {
        setBody0(src.declaringClass, src.methodInfo,
                 declaringClass, methodInfo, map);
    }

    
Inserts bytecode just after another constructor in the super class
or this class is called.
It does not work if this object represents a class initializer.

Params:
  • src –       the source code representing the inserted bytecode.
                 It must be a single statement or block.
/**
     * Inserts bytecode just after another constructor in the super class
     * or this class is called.
     * It does not work if this object represents a class initializer.
     *
     * @param src       the source code representing the inserted bytecode.
     *                  It must be a single statement or block.
     */
    public void insertBeforeBody(String src) throws CannotCompileException {
        CtClass cc = declaringClass;
        cc.checkModify();
        if (isClassInitializer())
            throw new CannotCompileException("class initializer");

        CodeAttribute ca = methodInfo.getCodeAttribute();
        CodeIterator iterator = ca.iterator();
        Bytecode b = new Bytecode(methodInfo.getConstPool(),
                                  ca.getMaxStack(), ca.getMaxLocals());
        b.setStackDepth(ca.getMaxStack());
        Javac jv = new Javac(b, cc);
        try {
            jv.recordParams(getParameterTypes(), false);
            jv.compileStmnt(src);
            ca.setMaxStack(b.getMaxStack());
            ca.setMaxLocals(b.getMaxLocals());
            iterator.skipConstructor();
            int pos = iterator.insertEx(b.get());
            iterator.insert(b.getExceptionTable(), pos);
            methodInfo.rebuildStackMapIf6(cc.getClassPool(), cc.getClassFile2());
        }
        catch (NotFoundException e) {
            throw new CannotCompileException(e);
        }
        catch (CompileError e) {
            throw new CannotCompileException(e);
        }
        catch (BadBytecode e) {
            throw new CannotCompileException(e);
        }
    }

    /* This method is called by addCatch() in CtBehavior.
     * super() and this() must not be in a try statement.
     */
    @Override
    int getStartPosOfBody(CodeAttribute ca) throws CannotCompileException {
        CodeIterator ci = ca.iterator();
        try {
            ci.skipConstructor();
            return ci.next();
        }
        catch (BadBytecode e) {
            throw new CannotCompileException(e);
        }
    }

    
Makes a copy of this constructor and converts it into a method.
The signature of the mehtod is the same as the that of this constructor.
The return type is void.  The resulting method must be
appended to the class specified by declaring.
If this constructor is a static initializer, the resulting method takes
no parameter.

An occurrence of another constructor call this()
or a super constructor call super() is
eliminated from the resulting method. 

The immediate super class of the class declaring this constructor
must be also a super class of the class declaring the resulting method.
If the constructor accesses a field, the class declaring the resulting method
must also declare a field with the same name and type.

Params:
  • name –              the name of the resulting method.
  • declaring –         the class declaring the resulting method.
/**
     * Makes a copy of this constructor and converts it into a method.
     * The signature of the mehtod is the same as the that of this constructor.
     * The return type is <code>void</code>.  The resulting method must be
     * appended to the class specified by <code>declaring</code>.
     * If this constructor is a static initializer, the resulting method takes
     * no parameter.
     *
     * <p>An occurrence of another constructor call <code>this()</code>
     * or a super constructor call <code>super()</code> is
     * eliminated from the resulting method. 
     *
     * <p>The immediate super class of the class declaring this constructor
     * must be also a super class of the class declaring the resulting method.
     * If the constructor accesses a field, the class declaring the resulting method
     * must also declare a field with the same name and type.
     *
     * @param name              the name of the resulting method.
     * @param declaring         the class declaring the resulting method.
     */
    public CtMethod toMethod(String name, CtClass declaring)
        throws CannotCompileException
    {
        return toMethod(name, declaring, null);
    }

    
Makes a copy of this constructor and converts it into a method.
The signature of the method is the same as the that of this constructor.
The return type is void.  The resulting method must be
appended to the class specified by declaring.
If this constructor is a static initializer, the resulting method takes
no parameter.

An occurrence of another constructor call this()
or a super constructor call super() is
eliminated from the resulting method. 

The immediate super class of the class declaring this constructor
must be also a super class of the class declaring the resulting method
(this is obviously true if the second parameter declaring is
the same as the class declaring this constructor).
If the constructor accesses a field, the class declaring the resulting method
must also declare a field with the same name and type.

Params:
  • name –              the name of the resulting method.
  • declaring –         the class declaring the resulting method.
                         It is normally the same as the class declaring this
                         constructor.
  • map –       the hash table associating original class names
                 with substituted names.  The original class names will be
                 replaced while making a copy.
                 map can be null.
/**
     * Makes a copy of this constructor and converts it into a method.
     * The signature of the method is the same as the that of this constructor.
     * The return type is <code>void</code>.  The resulting method must be
     * appended to the class specified by <code>declaring</code>.
     * If this constructor is a static initializer, the resulting method takes
     * no parameter.
     *
     * <p>An occurrence of another constructor call <code>this()</code>
     * or a super constructor call <code>super()</code> is
     * eliminated from the resulting method. 
     *
     * <p>The immediate super class of the class declaring this constructor
     * must be also a super class of the class declaring the resulting method
     * (this is obviously true if the second parameter <code>declaring</code> is
     * the same as the class declaring this constructor).
     * If the constructor accesses a field, the class declaring the resulting method
     * must also declare a field with the same name and type.
     *
     * @param name              the name of the resulting method.
     * @param declaring         the class declaring the resulting method.
     *                          It is normally the same as the class declaring this
     *                          constructor.
     * @param map       the hash table associating original class names
     *                  with substituted names.  The original class names will be
     *                  replaced while making a copy.
     *                  <code>map</code> can be <code>null</code>.
     */
    public CtMethod toMethod(String name, CtClass declaring, ClassMap map)
        throws CannotCompileException
    {
        CtMethod method = new CtMethod(null, declaring);
        method.copy(this, false, map);
        if (isConstructor()) {
            MethodInfo minfo = method.getMethodInfo2();
            CodeAttribute ca = minfo.getCodeAttribute();
            if (ca != null) {
                removeConsCall(ca);
                try {
                    methodInfo.rebuildStackMapIf6(declaring.getClassPool(),
                                                  declaring.getClassFile2());
                }
                catch (BadBytecode e) {
                    throw new CannotCompileException(e);
                }
            }
        }

        method.setName(name);
        return method;
    }

    private static void removeConsCall(CodeAttribute ca)
        throws CannotCompileException
    {
        CodeIterator iterator = ca.iterator();
        try {
            int pos = iterator.skipConstructor();
            if (pos >= 0) {
                int mref = iterator.u16bitAt(pos + 1);
                String desc = ca.getConstPool().getMethodrefType(mref);
                int num = Descriptor.numOfParameters(desc) + 1;
                if (num > 3)
                    pos = iterator.insertGapAt(pos, num - 3, false).position;

                iterator.writeByte(Opcode.POP, pos++);  // this
                iterator.writeByte(Opcode.NOP, pos);
                iterator.writeByte(Opcode.NOP, pos + 1);
                Descriptor.Iterator it = new Descriptor.Iterator(desc);
                while (true) {
                    it.next();
                    if (it.isParameter())
                        iterator.writeByte(it.is2byte() ? Opcode.POP2 : Opcode.POP,
                                           pos++);
                    else
                        break;
                }
            }
        }
        catch (BadBytecode e) {
            throw new CannotCompileException(e);
        }
    }
}