玩转javabytecode第一篇从java到asm的转换。

简书链接:玩转javabytecode第一篇从java到asm的转换。
文章字数：550,阅读全文大约需要2分钟
java源码

public class SimpleMain {
    public static void main(String[] args){
        System.out.println("333:");

    }
}

javap反汇编
javap -p -c -s -l TestAsmTrans.class

显示本地变量表 ```-c``` 反汇编

1 2	``` E:\eclipse_workspace\TestAsm\bin>javap -p -c -s -l TestAsmTrans.class

结果如下:

Compiled from "TestAsmTrans.java"
public class TestAsmTrans {
  public TestAsmTrans();
    descriptor: ()V
    Code:
       0: aload_0
       1: invokespecial #8                  // Method java/lang/Object."<init>":()V
       4: return
    LineNumberTable:
      line 2: 0
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0       5     0  this   LTestAsmTrans;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    Code:
       0: getstatic     #16                 // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #22                 // String 333:
       5: invokevirtual #24                 // Method java/io/PrintStream.println:(Ljava/lang/Stri
       8: return
    LineNumberTable:
      line 4: 0
      line 6: 8
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0       9     0  args   [Ljava/lang/String;
}

E:\eclipse_workspace\TestAsm\bin>

转换为更加容易看懂的代码
eclipse下载byptecode view插件

public class TestAsmTrans {

  // compiled from: TestAsmTrans.java

  // access flags 0x1
  public <init>()V
   L0
    LINENUMBER 2 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
    RETURN
   L1
    LOCALVARIABLE this LTestAsmTrans; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x9
  public static main([Ljava/lang/String;)V
   L0
    LINENUMBER 4 L0
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    LDC "333:"
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L1
    LINENUMBER 6 L1
    RETURN
   L2
    LOCALVARIABLE args [Ljava/lang/String; L0 L2 0
    MAXSTACK = 2
    MAXLOCALS = 1
}

asm转换。

L0就表示label0

ClassWriter classWriter = new ClassWriter(0);
FieldVisitor fieldVisitor;
MethodVisitor methodVisitor;
AnnotationVisitor annotationVisitor0;

classWriter.visit(V1_8, ACC_PUBLIC | ACC_SUPER, "test/SimpleMain", null, "java/lang/Object", null);

classWriter.visitSource("SimpleMain.java", null);

{
methodVisitor = classWriter.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
methodVisitor.visitCode();
Label label0 = new Label();
methodVisitor.visitLabel(label0);
methodVisitor.visitLineNumber(7, label0);
methodVisitor.visitVarInsn(ALOAD, 0);
methodVisitor.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
methodVisitor.visitInsn(RETURN);
Label label1 = new Label();
methodVisitor.visitLabel(label1);
methodVisitor.visitLocalVariable("this", "Ltest/SimpleMain;", null, label0, label1, 0);
methodVisitor.visitMaxs(1, 1);
methodVisitor.visitEnd();
}
{
methodVisitor = classWriter.visitMethod(ACC_PUBLIC | ACC_STATIC, "main", "([Ljava/lang/String;)V", null, null);
methodVisitor.visitCode();
Label label0 = new Label();
methodVisitor.visitLabel(label0);
methodVisitor.visitLineNumber(9, label0);
methodVisitor.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
methodVisitor.visitLdcInsn("333:");
methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
Label label1 = new Label();
methodVisitor.visitLabel(label1);
methodVisitor.visitLineNumber(11, label1);
methodVisitor.visitInsn(RETURN);
Label label2 = new Label();
methodVisitor.visitLabel(label2);
methodVisitor.visitLocalVariable("args", "[Ljava/lang/String;", null, label0, label2, 0);
methodVisitor.visitMaxs(2, 1);
methodVisitor.visitEnd();
}
classWriter.visitEnd();

return classWriter.toByteArray();
}
}

调用顺序

1
2
3

* visit visitSource? visitOuterClass? ( visitAnnotation | visitAttribute )*( visitInnerClass | visitField |
* visitMethod )*
* visitEnd

更多操作

java

class TestIntByte {
    @Test
    public void test() {
        System.out.println(new int[]{133333333, 66663, 4666663, 533333, 633333333});
    }

    @Test
    public void testCharByInt() {
        System.out.println(new char[]{(char) 5, (char) 6, (char) 7, (char) 333});
    }

    @Test
    public void testChar1() {
        System.out.println(new char[]{'b', 'd', 'n', 'a'});
    }

      @Test
    public void testLong() {
        System.out.println(new long[]{100l,600l,15550l,3330l});
    }


      @Test
    public void testFloat() {
        System.out.println(new float[]{100.0f,600f,15550f,3330f});
    }
}

asm

F:\src\git_project\RXJavaStudy\tools\asm-all-6.0_BETA>java -cp asm-6.0.jar org.objectweb.asm.util.ASMifier TestIntByte.class
import java.util.*;
import org.objectweb.asm.*;
public class TestIntByteDump implements Opcodes {

public static byte[] dump () throws Exception {

ClassWriter cw = new ClassWriter(0);
FieldVisitor fv;
MethodVisitor mv;
AnnotationVisitor av0;

cw.visit(V1_8, ACC_SUPER, "TestIntByte", null, "java/lang/Object", null);

{
mv = cw.visitMethod(0, "<init>", "()V", null, null);
mv.visitCode();
mv.visitVarInsn(ALOAD, 0);
mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
mv.visitInsn(RETURN);
mv.visitMaxs(1, 1);
mv.visitEnd();
}
{
mv = cw.visitMethod(ACC_PUBLIC, "test", "()V", null, null);
{
av0 = mv.visitAnnotation("Lorg/junit/Test;", true);
av0.visitEnd();
}
mv.visitCode();
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitInsn(ICONST_5);
mv.visitIntInsn(NEWARRAY, T_INT);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_0);
mv.visitLdcInsn(new Integer(133333333));
mv.visitInsn(IASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_1);
mv.visitLdcInsn(new Integer(66663));
mv.visitInsn(IASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_2);
mv.visitLdcInsn(new Integer(4666663));
mv.visitInsn(IASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_3);
mv.visitLdcInsn(new Integer(533333));
mv.visitInsn(IASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_4);
mv.visitLdcInsn(new Integer(633333333));
mv.visitInsn(IASTORE);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/Object;)V", false);
mv.visitInsn(RETURN);
mv.visitMaxs(5, 1);
mv.visitEnd();
}
{
mv = cw.visitMethod(ACC_PUBLIC, "testCharByInt", "()V", null, null);
{
av0 = mv.visitAnnotation("Lorg/junit/Test;", true);
av0.visitEnd();
}
mv.visitCode();
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitInsn(ICONST_4);
mv.visitIntInsn(NEWARRAY, T_CHAR);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_0);
mv.visitInsn(ICONST_5);
mv.visitInsn(CASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_1);
mv.visitIntInsn(BIPUSH, 6);
mv.visitInsn(CASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_2);
mv.visitIntInsn(BIPUSH, 7);
mv.visitInsn(CASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_3);
mv.visitIntInsn(SIPUSH, 333);
mv.visitInsn(CASTORE);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "([C)V", false);
mv.visitInsn(RETURN);
mv.visitMaxs(5, 1);
mv.visitEnd();
}
{
mv = cw.visitMethod(ACC_PUBLIC, "testChar1", "()V", null, null);
{
av0 = mv.visitAnnotation("Lorg/junit/Test;", true);
av0.visitEnd();
}
mv.visitCode();
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitInsn(ICONST_4);
mv.visitIntInsn(NEWARRAY, T_CHAR);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_0);
mv.visitIntInsn(BIPUSH, 98);
mv.visitInsn(CASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_1);
mv.visitIntInsn(BIPUSH, 100);
mv.visitInsn(CASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_2);
mv.visitIntInsn(BIPUSH, 110);
mv.visitInsn(CASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_3);
mv.visitIntInsn(BIPUSH, 97);
mv.visitInsn(CASTORE);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "([C)V", false);
mv.visitInsn(RETURN);
mv.visitMaxs(5, 1);
mv.visitEnd();
}
{
mv = cw.visitMethod(ACC_PUBLIC, "testLong", "()V", null, null);
{
av0 = mv.visitAnnotation("Lorg/junit/Test;", true);
av0.visitEnd();
}
mv.visitCode();
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitInsn(ICONST_4);
mv.visitIntInsn(NEWARRAY, T_LONG);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_0);
mv.visitLdcInsn(new Long(100L));
mv.visitInsn(LASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_1);
mv.visitLdcInsn(new Long(600L));
mv.visitInsn(LASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_2);
mv.visitLdcInsn(new Long(15550L));
mv.visitInsn(LASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_3);
mv.visitLdcInsn(new Long(3330L));
mv.visitInsn(LASTORE);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/Object;)V", false);
mv.visitInsn(RETURN);
mv.visitMaxs(6, 1);
mv.visitEnd();
}
{
mv = cw.visitMethod(ACC_PUBLIC, "testFloat", "()V", null, null);
{
av0 = mv.visitAnnotation("Lorg/junit/Test;", true);
av0.visitEnd();
}
mv.visitCode();
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitInsn(ICONST_4);
mv.visitIntInsn(NEWARRAY, T_FLOAT);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_0);
mv.visitLdcInsn(new Float("100.0"));
mv.visitInsn(FASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_1);
mv.visitLdcInsn(new Float("600.0"));
mv.visitInsn(FASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_2);
mv.visitLdcInsn(new Float("15550.0"));
mv.visitInsn(FASTORE);
mv.visitInsn(DUP);
mv.visitInsn(ICONST_3);
mv.visitLdcInsn(new Float("3330.0"));
mv.visitInsn(FASTORE);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/Object;)V", false);
mv.visitInsn(RETURN);
mv.visitMaxs(5, 1);
mv.visitEnd();
}
cw.visitEnd();

return cw.toByteArray();
}
}

更多技巧使用官网的工具
https://asm.ow2.io/javadoc/overview-summary.html

可以方便快捷的查看，可惜，我当时在公司研究忘记上传github了，当时是做了一个脚本来着
可以边写边预览源代码。