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Java字節碼深入解析

編輯：關於JAVA

一：Java字節代碼的組織形式

類文件{

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}

二：查看方法 --- Javap命令

例子：有一個Java類Demo.Java

public class Demo { 
    private String str1; 
    private String str2; 
    private int num1; 
    private int num2; 
    public static final String STATIC_DATA = "hello world"; 
     
    private void sayHello1(){ 
        System.out.println("this is method1..."); 
    } 
    private void sayHello2(){ 
        System.out.println("this is method2..."); 
    } 
    public void sayHello3(){ 
        System.out.println("this is method3..."); 
    } 
}

通過jdk自帶的反編譯工具命令 Javap 可以查看class文件的字節碼信息

D:\>Javap -verbose Demo >> Demo.txt

Demo.txt：

Compiled from "Demo.Java" 
public class Demo extends Java.lang.Object 
  SourceFile: "Demo.Java" 
  minor version: 0 
  major version: 49   
  
  Constant pool: 
const #1 = class      #2;   //  Demo 
const #2 = Asciz     Demo; 
const #3 = class      #4;   //  Java/lang/Object 
const #4 = Asciz     Java/lang/Object; 
const #5 = Asciz     str1; 
const #6 = Asciz     LJava/lang/String;; 
const #7 = Asciz     str2; 
const #8 = Asciz     num1; 
const #9 = Asciz     I; 
const #10 = Asciz   num2; 
const #11 = Asciz   STATIC_DATA; 
const #12 = Asciz   ConstantValue; 
const #13 = String  #14; //  hello world 
const #14 = Asciz   hello world; 
const #15 = Asciz   <init>; 
const #16 = Asciz   ()V; 
const #17 = Asciz   Code; 
const #18 = Method       #3.#19;   //  Java/lang/Object."<init>":()V 
const #19 = NameAndType    #15:#16;//  "<init>":()V 
const #20 = Asciz   LineNumberTable; 
const #21 = Asciz   LocalVariableTable; 
const #22 = Asciz   this; 
const #23 = Asciz   LDemo;; 
const #24 = Asciz   sayHello1; 
const #25 = FIEld   #26.#28;  //  java/lang/System.out:LJava/io/PrintStream; 
const #26 = class    #27; //  Java/lang/System 
const #27 = Asciz   Java/lang/System; 
const #28 = NameAndType    #29:#30;//  out:LJava/io/PrintStream; 
const #29 = Asciz   out; 
const #30 = Asciz   LJava/io/PrintStream;; 
const #31 = String  #32; //  this is method1... 
const #32 = Asciz   this is method1...; 
const #33 = Method       #34.#36;  //  java/io/PrintStream.println:(LJava/lang/String;)V 
const #34 = class    #35; //  Java/io/PrintStream 
const #35 = Asciz   Java/io/PrintStream; 
const #36 = NameAndType    #37:#38;//  println:(LJava/lang/String;)V 
const #37 = Asciz   println; 
const #38 = Asciz   (LJava/lang/String;)V; 
const #39 = Asciz   sayHello2; 
const #40 = String  #41; //  this is method2... 
const #41 = Asciz   this is method2...; 
const #42 = Asciz   sayHello3; 
const #43 = String  #44; //  this is method3... 
const #44 = Asciz   this is method3...; 
const #45 = Asciz   SourceFile; 
const #46 = Asciz   Demo.Java; 
  
{ 
public static final Java.lang.String STATIC_DATA; 
  Constant value: String hello world 
public Demo(); 
  Code: 
   Stack=1, Locals=1, Args_size=1 
   0:      aload_0 
   1:      invokespecial  #18; //Method Java/lang/Object."<init>":()V 
   4:      return 
  LineNumberTable: 
   line 2: 0 
  LocalVariableTable: 
   Start  Length  Slot  Name   Signature 
   0      5      0    this       LDemo; 
  
public void sayHello3(); 
  Code: 
   Stack=2, Locals=1, Args_size=1 
   0:      getstatic   #25; //FIEld java/lang/System.out:LJava/io/PrintStream; 
   3:      ldc   #43; //String this is method3... 
   5:      invokevirtual  #33; //Method java/io/PrintStream.println:(LJava/lang/String;)V 
   8:      return 
  LineNumberTable: 
   line 17: 0 
   line 18: 8 
  LocalVariableTable: 
   Start  Length  Slot  Name   Signature 
   0      9      0    this       LDemo; 
}

解析：

1.版本號 major version: 49 //Java版本 jdk1.6顯示的是50， jdk1.5顯示的是49，jdk1.4顯示的是58 ，高版本能執行低版本的class文件

2.常量池Constant pool

Method：方法

FIEld：字段

String：字符串

Asciz：簽名如<init>由jvm調用，其他是不能夠去調用它的

NameAndType：變量名的類型

Class：類

通過字節碼，我們可以看到Demo類繼承於Java.lang.Object，如果類中沒有顯式聲明構造函數的話，編譯器會插入一個缺省無參的構造函數(構造函數在JVM級別是顯示成<init>的普通函數)。

三：檢測代碼的效率問題

學習Java的過程中，都會了解到字符串合並時要用到StringBuffer 來代替String，那下面就來通過Java字節碼來驗證兩種方式的效率性。

例子：一個Java類 TestString.Java

<strong>public class TestString { 
    public String testString(String str1, String str2){ 
       return str1 + str2; 
    } 
    public String testStringBuffer(StringBuffer sb, String str){ 
       return sb.append(str).toString(); 
    } 
} 
 </strong>

Javap –c TestString 後字節碼信息：

Compiled from "TestString.Java" 
public class TestString extends Java.lang.Object{ 
public TestString(); 
  Code: 
   0:      aload_0 
   1:      invokespecial  #8; //Method Java/lang/Object."<init>":()V 
   4:      return 
  
public java.lang.String testString(java.lang.String, Java.lang.String); 
  Code: 
   0:      new #16; //class Java/lang/StringBuilder 
   3:      dup 
   4:      aload_1 
   5:      invokestatic    #18; //Method java/lang/String.valueOf:(Ljava/lang/Object;)LJava/lang/String; 
   8:      invokespecial  #24; //Method java/lang/StringBuilder."<init>":(LJava/lang/String;)V 
   11:     aload_2 
   12:    invokevirtual  #27; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)LJava/lang/StringBuilder; 
   15:    invokevirtual  #31; //Method java/lang/StringBuilder.toString:()LJava/lang/String; 
   18:    areturn 
  
public java.lang.String testStringBuffer(java.lang.StringBuffer, Java.lang.String); 
  Code: 
   0:      aload_1 
   1:      aload_2 
   2:      invokevirtual  #40; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)LJava/lang/StringBuffer; 
   5:      invokevirtual  #45; //Method java/lang/StringBuffer.toString:()LJava/lang/String; 
   8:      areturn 
}

從上面編譯後的字節碼信息可以看出來，方法testString 調用了五個方法：new 、invokestatic 、invokespecial 和兩個invokevirtual ; 而testStringBuffer 方法只調用了兩個invokevirtual 方法。第一個方法比第二個方法多做了好多工作，其效率當然是要低的。而且我們從java/lang/StringBuilder.append:(Ljava/lang/String;)LJava/lang/StringBuilder;

可以看出來其實對於String字符串合並，內部還是轉化為StringBuilder的方法調用，這是因為String是長度不可變的，所以不如直接采用StringBuilder（與StringBuffer 長度都是可變的，只不過前者是非線程安全，後者是線程安全）進行字符串合並。

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