結(jié)構(gòu)化鎖定是指方法調(diào)用期間每一個(gè)monitor退出都與前面monitor進(jìn)入相匹配的情形。JVM通過以下兩條規(guī)則來保證結(jié)結(jié)構(gòu)化鎖成立(T代表一線程，M代表一個(gè)monitor)：

   1)T在方法執(zhí)行時(shí)持有M的次數(shù)必須與T在方法完成時(shí)釋放的M次數(shù)相等
   2)任何時(shí)刻都不會(huì)出現(xiàn)T釋放M的次數(shù)比T持有M的次數(shù)多的情況
    

| 前/后綴 | 操作數(shù)類型 |
| :------ | :------ |
| i | 整數(shù) | 
| l | 長整數(shù) | 
| s | 短整數(shù) | 
| b | 字節(jié) | 
| c | 字符 | 
| f | 單精度浮點(diǎn)數(shù) | 
| d | 雙精度浮點(diǎn)數(shù) | 
| z | 布爾值 | 
| a | 引用 | 

引用自深入理解java虛擬機(jī) 字節(jié)碼指令簡介

JAVA源碼

package com.taobao.film;

/**
 * @author - zhupin(kaiqiang.gkq@alibaba-inc.com)
 */
public class Demo {
    private static final String HELLO_CONST = "Hello";
    private static String CONST = null;

    static {
        CONST = HELLO_CONST + "%s!";
    }

    public static void main(String[] args) {
        if (args != null && args.length == 1) {
            System.out.println(String.format(CONST, args[0]));
        }
    }
}

對應(yīng)字節(jié)碼
簡記 ms:操作數(shù)棧最大深度 ml:局部變量表最大容量 s:操作數(shù)棧 l:局部變量表

// class version 49.0 (49)
// access flags 0x21
public class com/taobao/film/Demo {

  // compiled from: Demo.java

  // access flags 0x1A 常量,類被裝載時(shí)分配空間 private static final String HELLO_CONST = "Hello";
  private final static Ljava/lang/String; HELLO_CONST = "Hello"

  // access flags 0xA 靜態(tài)屬性,類被裝載時(shí)分配空間 private static String CONST ;
  private static Ljava/lang/String; CONST

  // access flags 0x1 沒有重載構(gòu)造函數(shù)，默認(rèn)構(gòu)造函數(shù)()V
  public ()V
   L0 // 標(biāo)簽表示方法的字節(jié)碼中的位置。標(biāo)簽用于跳轉(zhuǎn)，goto和切換指令，以及用于嘗試catch塊。標(biāo)簽指定剛剛之后的指令。注意，在標(biāo)簽和它指定的指令（例如其他標(biāo)簽，堆棧映射幀，行號等）之間可以有其他元素。
    LINENUMBER 6 L0 //異常的棧信息中對應(yīng)的line number，可刪除不影響運(yùn)行
    ALOAD 0 //this
    INVOKESPECIAL java/lang/Object. ()V //this.super()
    RETURN
   L1
    LOCALVARIABLE this Lcom/taobao/film/Demo; L0 L1 0
    MAXSTACK = 1 //最大棧深度1,壓棧局部變量this
    MAXLOCALS = 1 //局部變量數(shù)1,局部變量this

  // access flags 0x9 簡記 ms:操作數(shù)棧最大深度 ml:局部變量表最大容量 s:操作數(shù)棧 l:局部變量表
  public static main([Ljava/lang/String;)V
   L0
    LINENUMBER 15 L0
    ALOAD 0 //非靜態(tài)方法,局部變量0即方法入?yún)⒌谝粋€(gè)參數(shù)args reference; ms=1,ml=1,s=[args ref],l=[args ref]
    IFNULL L1  //if(args ref==null)跳轉(zhuǎn) L1 return
    ALOAD 0 //load args ms=1,ml=1,s=[args ref],l=[args ref]
    ARRAYLENGTH//計(jì)算args.length將結(jié)果入操作數(shù)棧 ms=1,ml=1,s=[args ref],l=[args ref] 
    ICONST_1 //數(shù)字常量1 ms=2,ml=1,s=[1,args ref],l=[args ref] 
    IF_ICMPNE L1 //if(args.length == 1) 跳轉(zhuǎn) L1 return 
   L2 //ms=2,ml=1,s=[],l=[args ref] 
    LINENUMBER 16 L2
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;//訪問System.out，入操作數(shù)棧 ms=2,ml=1,s=[System.out],l=[args ref]  
    GETSTATIC com/taobao/film/Demo.CONST : Ljava/lang/String;//訪問CONST，入操作數(shù)棧 ms=2,ml=1,s=[CONST,System.out],l=[args ref]  
    {//這里一段，實(shí)現(xiàn)的是new Object[]{args[0]},String.format(String format, Object... args) 
    ICONST_1//1  ms=3,ml=1,s=[1,CONST,System.out],l=[args ref]  
    ANEWARRAY java/lang/Object// new Object[1]; ms=3,ml=1,s=[objects ref,CONST,System.out],l=[args ref]  
    DUP // ms=4,ml=1,s=[objects ref,objects ref,CONST,System.out],l=[args ref]  
    ICONST_0//0 ms=5,ml=1,s=[0,objects ref,objects ref,CONST,System.out],l=[args ref]  
    ALOAD 0 //args reference入操作數(shù)棧; ms=6,ml=1,s=[args ref,0,objects ref,objects ref,CONST,System.out],l=[args ref]  
    ICONST_0 //0 ms=7,ml=1,s=[0,args ref,0,objects ref,objects ref,CONST,System.out],l=[args ref] 
    AALOAD //args[0] ms=7,ml=1,s=[args[0],0,objects ref,objects ref,CONST,System.out],l=[args ref]
    AASTORE //objects[0]=args[0]; ms=7,ml=1,s=[objects ref,CONST,System.out],l=[args ref]
    }
    INVOKESTATIC java/lang/String.format (Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/String;//String.format(CONST,objects) ms=7,ml=1,s=[objects ref,CONST,System.out],l=[args ref]
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V //System.out.println(...); ms=7,ml=1,s=[],l=[args ref]
   L1
    LINENUMBER 18 L1
    RETURN
   L3
    LOCALVARIABLE args [Ljava/lang/String; L0 L3 0
    MAXSTACK = 7
    MAXLOCALS = 1

  // access flags 0x8 V classload init，靜態(tài)代碼初始塊，類被裝在時(shí)init
  static ()V
   L0
    LINENUMBER 8 L0 
    ACONST_NULL //壓棧常量NULL
    PUTSTATIC com/taobao/film/Demo.CONST : Ljava/lang/String; //靜態(tài)屬性賦值，CONST=null
   L1
    LINENUMBER 11 L1
    LDC "Hello%s!" //加載常量Hello%s! 編譯優(yōu)化，hello+%s編譯優(yōu)化為常量Hello%s!
    PUTSTATIC com/taobao/film/Demo.CONST : Ljava/lang/String;//賦值Hello%s!
   L2
    LINENUMBER 12 L2
    RETURN
    MAXSTACK = 1 //只有賦值操作，操作數(shù)棧深度1
    MAXLOCALS = 0 //靜態(tài)方法無局部變量
}

回答這個(gè)問題前我們首先得明白字節(jié)碼是作用于運(yùn)行期，所以一般用來在運(yùn)行期改變類的行為，基本上都會(huì)結(jié)合代理模式使用。

ASM

BCEL

CGLIB

Javassist

Byte Buddy

本文來介紹下ASM框架的使用(了解一點(diǎn)底層字節(jié)碼操作，對了解JVM執(zhí)行過程以及我們常用框架的實(shí)現(xiàn)原理都會(huì)有新的認(rèn)識(shí))

ClassReader用來讀取原有的字節(jié)碼，ClassWriter用于寫入字節(jié)碼，ClassVisitor、FieldVisitor、MethodVisitor、AnnotationVisitor訪問修改對應(yīng)組件。(一般不要通過ClassReader讀取再通過Vistor修改類型為再ClassWriter回寫，覆蓋原有類行為，采用繼承會(huì)更安全)

注:

Diving into Bytecode Manipulation

[字節(jié)碼操縱技術(shù)探秘
](http://www.infoq.com/cn/artic...

以下代碼效果等同于上面的Demo示例

import java.io.FileOutputStream;
import java.lang.reflect.Method;

import org.objectweb.asm.AnnotationVisitor;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.FieldVisitor;
import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

public class DemoDump implements Opcodes {

    public static byte[] dump() throws Exception {

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

        cw.visit(V1_5, ACC_PUBLIC + ACC_SUPER, "com/taobao/film/Demo", null, "java/lang/Object", null);

        cw.visitSource("Demo.java", null);

        {
            fv = cw.visitField(ACC_PRIVATE + ACC_FINAL + ACC_STATIC, "HELLO_CONST", "Ljava/lang/String;", null,
                "Hello");
            fv.visitEnd();
        }
        {
            fv = cw.visitField(ACC_PRIVATE + ACC_STATIC, "CONST", "Ljava/lang/String;", null, null);
            fv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC, "", "()V", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(6, l0);
            mv.visitVarInsn(ALOAD, 0);
            mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "", "()V", false);
            mv.visitInsn(RETURN);
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLocalVariable("this", "Lcom/taobao/film/Demo;", null, l0, l1, 0);
            mv.visitMaxs(1, 1);
            mv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC + ACC_STATIC, "main", "([Ljava/lang/String;)V", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(15, l0);
            mv.visitVarInsn(ALOAD, 0);
            Label l1 = new Label();
            mv.visitJumpInsn(IFNULL, l1);
            mv.visitVarInsn(ALOAD, 0);
            mv.visitInsn(ARRAYLENGTH);
            mv.visitInsn(ICONST_1);
            mv.visitJumpInsn(IF_ICMPNE, l1);
            Label l2 = new Label();
            mv.visitLabel(l2);
            mv.visitLineNumber(16, l2);
            mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
            mv.visitFieldInsn(GETSTATIC, "com/taobao/film/Demo", "CONST", "Ljava/lang/String;");
            mv.visitInsn(ICONST_1);
            mv.visitTypeInsn(ANEWARRAY, "java/lang/Object");
            mv.visitInsn(DUP);
            mv.visitInsn(ICONST_0);
            mv.visitVarInsn(ALOAD, 0);
            mv.visitInsn(ICONST_0);
            mv.visitInsn(AALOAD);
            mv.visitInsn(AASTORE);
            mv.visitMethodInsn(INVOKESTATIC, "java/lang/String", "format",
                "(Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/String;", false);
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
            mv.visitLabel(l1);
            mv.visitLineNumber(18, l1);
            mv.visitInsn(RETURN);
            Label l3 = new Label();
            mv.visitLabel(l3);
            mv.visitLocalVariable("args", "[Ljava/lang/String;", null, l0, l3, 0);
            mv.visitMaxs(7, 1);
            mv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_STATIC, "", "()V", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(8, l0);
            mv.visitInsn(ACONST_NULL);
            mv.visitFieldInsn(PUTSTATIC, "com/taobao/film/Demo", "CONST", "Ljava/lang/String;");
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLineNumber(11, l1);
            mv.visitLdcInsn("Hello%s!");
            mv.visitFieldInsn(PUTSTATIC, "com/taobao/film/Demo", "CONST", "Ljava/lang/String;");
            Label l2 = new Label();
            mv.visitLabel(l2);
            mv.visitLineNumber(12, l2);
            mv.visitInsn(RETURN);
            mv.visitMaxs(1, 0);
            mv.visitEnd();
        }
        cw.visitEnd();

        return cw.toByteArray();
    }

    public static class MyClassLoader extends ClassLoader {
        public MyClassLoader() {
            super();
        }

        public MyClassLoader(ClassLoader cl) {
            super(cl);
        }

        public Class defineClass(String name, byte[] b) {
            return defineClass(name, b, 0, b.length);
        }
    }

    public static void main(String[] args) throws Exception {
        byte[] bytes = dump();
        FileOutputStream fileOutputStream = new FileOutputStream("D:Demo.class");
        fileOutputStream.write(bytes);
        fileOutputStream.close();
        MyClassLoader classLoader = new MyClassLoader(Thread.currentThread().getContextClassLoader());
        Class testClass = classLoader.defineClass("com.taobao.film.Demo",
            bytes);
        // invoke static
        Method staticMain = testClass.getMethod("main", String[].class);
        staticMain.invoke(null, new Object[] {new String[] {"zhupin"}});
    }
}

XML快速序列化

為無狀態(tài)邏輯類的指定函數(shù)產(chǎn)生一個(gè)代理，代理接口接受字符串?dāng)?shù)組，轉(zhuǎn)換后調(diào)用原函數(shù)

fastjson hotcode asm

dubbo hsf javaassist

hibernate cglib javaassist

spring aop cglib (通過設(shè)置-Dcglib.debugLocation=D://tmp 開啟classdump)

mockito cglib

最早接觸字節(jié)碼編程還是在使用IL(C# Emit)，也在大型項(xiàng)目中使用過ASM獲得了不錯(cuò)的效果。不過個(gè)人覺得操作字節(jié)碼編程還是要保持謹(jǐn)慎態(tài)度。下面是我的一點(diǎn)小建議：

操作字節(jié)碼作用于運(yùn)行期，對于開發(fā)人員是完全透明的。

字節(jié)碼編程的可閱讀可維護(hù)性比較差，不要濫用字節(jié)碼編程。

能通過設(shè)計(jì)模式實(shí)現(xiàn)的場景盡量通過設(shè)計(jì)模式實(shí)現(xiàn)；

字節(jié)碼編程中復(fù)雜的邏輯也盡量使用java實(shí)現(xiàn)，在字節(jié)碼中調(diào)用；

使用字節(jié)碼解決一些框架性的問題，不要用于處理易變邏輯；

字節(jié)碼編程從邏輯塊著手，優(yōu)先明確程序跳轉(zhuǎn)Label，再補(bǔ)充邏輯執(zhí)行

借助工具
推薦使用intellj idea插件ASM Bytecode Outline，目前生成的字節(jié)碼對應(yīng)到 ASM 5.x。

使用decompile工具校驗(yàn)生成代碼是否正確。

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