プログラムの変更と、何を変更する方がよいか：プログラムの実行可能コードまたはAST？

import java.awt.*; import java.applet.*; public class DocFooter extends Applet { String date; String email; public void init() { resize(500,100); date = getParameter("LAST_UPDATED"); email = getParameter("EMAIL"); } public void paint(Graphics g) { g.drawString(date + " by ",100, 15); g.drawString(email,290,15); } } 

 Compiled from "DocFooter.java" public class DocFooter extends java.applet.Applet { java.lang.String date; java.lang.String email; public DocFooter(); Code: 0: aload_0 1: invokespecial #1 // Method java/applet/Applet."<init>":()V 4: return public void init(); Code: 0: aload_0 1: sipush 500 4: bipush 100 6: invokevirtual #2 // Method resize:(II)V 9: aload_0 10: aload_0 11: ldc #3 // String LAST_UPDATED 13: invokevirtual #4 // Method getParameter:(Ljava/lang/String;)Ljava/lang/String; 16: putfield #5 // Field date:Ljava/lang/String; 19: aload_0 20: aload_0 21: ldc #6 // String EMAIL 23: invokevirtual #4 // Method getParameter:(Ljava/lang/String;)Ljava/lang/String; 26: putfield #7 // Field email:Ljava/lang/String; 29: return public void paint(java.awt.Graphics); Code: 0: aload_1 1: new #8 // class java/lang/StringBuilder 4: dup 5: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V 8: aload_0 9: getfield #5 // Field date:Ljava/lang/String; 12: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 15: ldc #11 // String by 17: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 20: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 23: bipush 100 25: bipush 15 27: invokevirtual #13 // Method java/awt/Graphics.drawString:(Ljava/lang/String;II)V 30: aload_1 31: aload_0 32: getfield #7 // Field email:Ljava/lang/String; 35: sipush 290 38: bipush 15 40: invokevirtual #13 // Method java/awt/Graphics.drawString:(Ljava/lang/String;II)V 43: return } 

 /*** * ASM examples: examples showing how ASM can be used * Copyright (c) 2000-2011 INRIA, France Telecom * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * 3. Neither the name of the copyright holders nor the names of its * contributors may be used to endorse or promote products derived from * this software without specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF * THE POSSIBILITY OF SUCH DAMAGE. */ import java.util.HashSet; import java.util.Iterator; import java.util.List; import java.util.Set; import org.objectweb.asm.ClassReader; import org.objectweb.asm.MethodVisitor; import org.objectweb.asm.Opcodes; import org.objectweb.asm.tree.AbstractInsnNode; import org.objectweb.asm.tree.ClassNode; import org.objectweb.asm.tree.IincInsnNode; import org.objectweb.asm.tree.MethodNode; import org.objectweb.asm.tree.VarInsnNode; import org.objectweb.asm.tree.analysis.Analyzer; import org.objectweb.asm.tree.analysis.BasicValue; import org.objectweb.asm.tree.analysis.BasicVerifier; import org.objectweb.asm.tree.analysis.Frame; import org.objectweb.asm.tree.analysis.SourceInterpreter; import org.objectweb.asm.tree.analysis.SourceValue; import org.objectweb.asm.util.TraceMethodVisitor; import org.objectweb.asm.util.Textifier; /** * @author Eric Bruneton */ public class Analysis implements Opcodes { public static void main(final String[] args) throws Exception { ClassReader cr = new ClassReader("Analysis"); ClassNode cn = new ClassNode(); cr.accept(cn, ClassReader.SKIP_DEBUG); List<MethodNode> methods = cn.methods; for (int i = 0; i < methods.size(); ++i) { MethodNode method = methods.get(i); if (method.instructions.size() > 0) { if (!analyze(cn, method)) { Analyzer<?> a = new Analyzer<BasicValue>( new BasicVerifier()); try { a.analyze(cn.name, method); } catch (Exception ignored) { } final Frame<?>[] frames = a.getFrames(); Textifier t = new Textifier() { @Override public void visitMaxs(final int maxStack, final int maxLocals) { for (int i = 0; i < text.size(); ++i) { StringBuilder s = new StringBuilder( frames[i] == null ? "null" : frames[i].toString()); while (s.length() < Math.max(20, maxStack + maxLocals + 1)) { s.append(' '); } System.err.print(Integer.toString(i + 1000) .substring(1) + " " + s + " : " + text.get(i)); } System.err.println(); } }; MethodVisitor mv = new TraceMethodVisitor(t); for (int j = 0; j < method.instructions.size(); ++j) { Object insn = method.instructions.get(j); ((AbstractInsnNode) insn).accept(mv); } mv.visitMaxs(0, 0); } } } } /* * Detects unused xSTORE instructions, ie xSTORE instructions without at * least one xLOAD corresponding instruction in their successor instructions * (in the control flow graph). */ public static boolean analyze(final ClassNode c, final MethodNode m) throws Exception { Analyzer<SourceValue> a = new Analyzer<SourceValue>( new SourceInterpreter()); Frame<SourceValue>[] frames = a.analyze(c.name, m); // for each xLOAD instruction, we find the xSTORE instructions that can // produce the value loaded by this instruction, and we put them in // 'stores' Set<AbstractInsnNode> stores = new HashSet<AbstractInsnNode>(); for (int i = 0; i < m.instructions.size(); ++i) { AbstractInsnNode insn = m.instructions.get(i); int opcode = insn.getOpcode(); if ((opcode >= ILOAD && opcode <= ALOAD) || opcode == IINC) { int var = opcode == IINC ? ((IincInsnNode) insn).var : ((VarInsnNode) insn).var; Frame<SourceValue> f = frames[i]; if (f != null) { Set<AbstractInsnNode> s = f.getLocal(var).insns; Iterator<AbstractInsnNode> j = s.iterator(); while (j.hasNext()) { insn = j.next(); if (insn instanceof VarInsnNode) { stores.add(insn); } } } } } // we then find all the xSTORE instructions that are not in 'stores' boolean ok = true; for (int i = 0; i < m.instructions.size(); ++i) { AbstractInsnNode insn = m.instructions.get(i); int opcode = insn.getOpcode(); if (opcode >= ISTORE && opcode <= ASTORE) { if (!stores.contains(insn)) { ok = false; System.err.println("method " + m.name + ", instruction " + i + ": useless store instruction"); } } } return ok; } /* * Test for the above method, with three useless xSTORE instructions. */ public int test(int i, int j) { i = i + 1; // ok, because i can be read after this point if (j == 0) { j = 1; // useless } else { try { j = j - 1; // ok, because j can be accessed in the catch int k = 0; if (i > 0) { k = i - 1; } return k; } catch (Exception e) { // useless ASTORE (e is never used) j = j + 1; // useless } } return 0; } } 

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