Java字節碼深入解析 || 使用Intellij idea如何快速查看Java類字節碼

2019-11-14 11:01:24

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java字節碼深入解析

一：Java字節代碼的組織形式

　　類文件{

　　OxCAFEBABE，小版本號，大版本號，常量池大小，常量池數組，訪問控制標記，當前類信息，父類信息，實現的接口個數，實現的接口信息數組，域個數，域信息數組，方法個數，方法信息數組，屬性個數，屬性信息數組

　　}

　　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類繼承于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并時要用到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 后字節碼信息：

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 }

　　從上面編譯后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；

　　可以看出來其實對于String字符串合并，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，后者是線程安全）進行字符串合并。