expreditor.java

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/*
 * Javassist, a Java-bytecode translator toolkit.
 * Copyright (C) 1999-2003 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.
 *
 * 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.expr;

import javassist.bytecode.*;
import javassist.CtClass;
import javassist.CannotCompileException;

/**
 * A translator of method bodies.
 *
 * <p>The users can define a subclass of this class to customize how to
 * modify a method body.  The overall architecture is similar to the
 * strategy pattern.
 *
 * <p>If <code>instrument()</code> is called in
 * <code>CtMethod</code>, the method body is scanned from the beginning
 * to the end.
 * Whenever an expression, such as a method call and a <tt>new</tt>
 * expression (object creation),
 * is found, <code>edit()</code> is called in <code>ExprEdit</code>.
 * <code>edit()</code> can inspect and modify the given expression.
 * The modification is reflected on the original method body.  If
 * <code>edit()</code> does nothing, the original method body is not
 * changed.
 *
 * <p>The following code is an example:
 *
 * <ul><pre>
 * CtMethod cm = ...;
 * cm.instrument(new ExprEditor() {
 *     public void edit(MethodCall m) throws CannotCompileException {
 *         if (m.getClassName().equals("Point")) {
 *             System.out.println(m.getMethodName() + " line: "
 *                                + m.getLineNumber());
 *     }
 * });
 * </pre></ul>
 *
 * <p>This code inspects all method calls appearing in the method represented
 * by <code>cm</code> and it prints the names and the line numbers of the
 * methods declared in class <code>Point</code>.  This code does not modify
 * the body of the method represented by <code>cm</code>.  If the method
 * body must be modified, call <code>replace()</code>
 * in <code>MethodCall</code>.
 *
 * @see javassist.CtClass#instrument(ExprEditor)
 * @see javassist.CtMethod#instrument(ExprEditor)
 * @see javassist.CtConstructor#instrument(ExprEditor)
 * @see MethodCall
 * @see NewExpr
 * @see FieldAccess
 *
 * @see javassist.CodeConverter
 */
public class ExprEditor {
    /**
     * Default constructor.  It does nothing.
     */
    public ExprEditor() {}

    static class NewOp {
        NewOp next;
        int pos;
        String type;

        NewOp(NewOp n, int p, String t) {
            next = n;
            pos = p;
            type = t;
        }
    }

    /**
     * Undocumented method.  Do not use; internal-use only.
     */
    public boolean doit(CtClass clazz, MethodInfo minfo)
        throws CannotCompileException
    {
        CodeAttribute codeAttr = minfo.getCodeAttribute();
        if (codeAttr == null)
            return false;

        CodeIterator iterator = codeAttr.iterator();
        boolean edited = false;
        int maxLocals = codeAttr.getMaxLocals();
        int maxStack = 0;

        NewOp newList = null;
        ConstPool cp = minfo.getConstPool();
        int prevOp = Opcode.NOP;

        while (iterator.hasNext())
            try {
                Expr expr = null;
                int pos = iterator.next();
                int c = iterator.byteAt(pos);

                if (c == Opcode.INVOKESTATIC || c == Opcode.INVOKEINTERFACE
                    || c == Opcode.INVOKEVIRTUAL) {
                    expr = new MethodCall(pos, iterator, clazz, minfo);
                    edit((MethodCall)expr);
                }
                else if (c == Opcode.GETFIELD || c == Opcode.GETSTATIC
                        || c == Opcode.PUTFIELD || c == Opcode.PUTSTATIC) {
                    expr = new FieldAccess(pos, iterator, clazz, minfo, c, prevOp);
                    edit((FieldAccess)expr);
                }
                else if (c == Opcode.NEW) {
                    int index = iterator.u16bitAt(pos + 1);
                    newList = new NewOp(newList, pos,
                                        cp.getClassInfo(index));
                }
                else if (c == Opcode.INVOKESPECIAL) {
                    if (newList != null && cp.isConstructor(newList.type,
                                iterator.u16bitAt(pos + 1)) > 0) {
                        expr = new NewExpr(pos, iterator, clazz, minfo,
                                           newList.type, newList.pos);
                        edit((NewExpr)expr);
                        newList = newList.next;
                    }
                    else {
                        expr = new MethodCall(pos, iterator, clazz, minfo);
                        MethodCall mcall = (MethodCall)expr;
                        if (!mcall.getMethodName().equals(
                                                MethodInfo.nameInit))
                            edit(mcall);
                    }
                }
                else if (c == Opcode.INSTANCEOF) {
                    expr = new Instanceof(pos, iterator, clazz, minfo);
                    edit((Instanceof)expr);
                }
                else if (c == Opcode.CHECKCAST) {
                    expr = new Cast(pos, iterator, clazz, minfo);
                    edit((Cast)expr);
                }
                prevOp = c;
                if (expr != null && expr.edited()) {
                    edited = true;
                    maxLocals = max(maxLocals, expr.locals());
                    maxStack = max(maxStack, expr.stack());
                }
            }
            catch (BadBytecode e) {
                throw new CannotCompileException(e);
            }

        ExceptionTable et = codeAttr.getExceptionTable();
        int n = et.size();
        for (int i = 0; i < n; ++i) {
            Handler h = new Handler(et, i, iterator, clazz, minfo);
            edit(h);
            if (h.edited()) {
                edited = true;
                maxLocals = max(maxLocals, h.locals());
                maxStack = max(maxStack, h.stack());
            }
        }

        codeAttr.setMaxLocals(maxLocals);
        codeAttr.setMaxStack(codeAttr.getMaxStack() + maxStack);
        return edited;
    }

    private int max(int i, int j) {
        return i > j ? i : j;
    }

    /**
     * Edits a <tt>new</tt> expression (overridable).
     * The default implementation performs nothing.
     *
     * @param e         the <tt>new</tt> expression creating an object.
     */
    public void edit(NewExpr e) throws CannotCompileException {}

    /**
     * Edits a method call (overridable).
     * The default implementation performs nothing.
     */
    public void edit(MethodCall m) throws CannotCompileException {}

    /**
     * Edits a field-access expression (overridable).
     * Field access means both read and write.
     * The default implementation performs nothing.
     */
    public void edit(FieldAccess f) throws CannotCompileException {}

    /**
     * Edits an instanceof expression (overridable).
     * The default implementation performs nothing.
     */
    public void edit(Instanceof i) throws CannotCompileException {}

    /**
     * Edits an expression for explicit type casting (overridable).
     * The default implementation performs nothing.
     */
    public void edit(Cast c) throws CannotCompileException {}

    /**
     * Edits a catch clause (overridable).
     * The default implementation performs nothing.
     */
    public void edit(Handler h) throws CannotCompileException {}
}