Java字節(jié)碼深入解析 || 使用Intellij idea如何快速查看Java類字節(jié)碼

2019-11-14 11:32:33

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java字節(jié)碼深入解析

一：Java字節(jié)代碼的組織形式

　　類文件{

　　OxCAFEBABE，小版本號(hào)，大版本號(hào)，常量池大小，常量池?cái)?shù)組，訪問控制標(biāo)記，當(dāng)前類信息，父類信息，實(shí)現(xiàn)的接口個(gè)數(shù)，實(shí)現(xiàn)的接口信息數(shù)組，域個(gè)數(shù)，域信息數(shù)組，方法個(gè)數(shù)，方法信息數(shù)組，屬性個(gè)數(shù)，屬性信息數(shù)組

　　}

　　二：查看方法 --- javap命令

　　例子：有一個(gè)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(guò)jdk自帶的反編譯工具命令 javap 可以查看class文件的字節(jié)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(hào) 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(zhí)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(diào)用，其他是不能夠去調(diào)用它的

　　NameAndType：變量名的類型

　　Class：類

　　通過(guò)字節(jié)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(gòu)造函數(shù)的話，編譯器會(huì)插入一個(gè)缺省無(wú)參的構(gòu)造函數(shù)(構(gòu)造函數(shù)在JVM級(jí)別是顯示成<init>的普通函數(shù))。

　　三：檢測(cè)代碼的效率問題

　　學(xué)習(xí)Java的過(guò)程中，都會(huì)了解到字符串合并時(shí)要用到StringBuffer 來(lái)代替String，那下面就來(lái)通過(guò)Java字節(jié)碼來(lái)驗(yàn)證兩種方式的效率性。

　　例子：一個(gè)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(jié)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(jié)碼信息可以看出來(lái)，方法testString 調(diào)用了五個(gè)方法：new 、invokestatic 、invokespecial 和兩個(gè)invokevirtual ；而testStringBuffer 方法只調(diào)用了兩個(gè)invokevirtual 方法。第一個(gè)方法比第二個(gè)方法多做了好多工作，其效率當(dāng)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(lái)其實(shí)對(duì)于String字符串合并，內(nèi)部還是轉(zhuǎn)化為StringBuilder的方法調(diào)用，這是因?yàn)镾tring是長(zhǎng)度不可變的，所以不如直接采用StringBuilder（與StringBuffer 長(zhǎng)度都是可變的，只不過(guò)前者是非線程安全，后者是線程安全）進(jìn)行字符串合并。