Java class 文件信息

在谈论JVM-运行时数据区时，我们遇到了一个方法区，关于方法区的信息都来自于class，在JDK的工具中，有一个javap命令可以用来显示class文件的信息。

查看class文件内容的命令如下：

javap -v XXX.clsss

在这里以TestClass.java示例

/** *  */package com.hx;import com.hx.vo.BaseParam;/** * @author MichaelKoo *  *         2017-7-12 */public class TestClass extends BaseParam implements Runnable {    private String name;    private static final int AGE = 18;    public void run() {        name = "MichaelKoo," + AGE;        try {            CsdnUtil.getContent(getClass());        } catch (Exception e) {            e.printStackTrace();        }    }}

编译成TestClass.class之后，使用javap命令打开信息如下：

Classfile /I:/adt_eclipse_work/HuanXinTest/bin/com/hx/TestClass.class  Last modified 2017-7-12; size 785 bytes  MD5 checksum 92698cfe5ce5c5322fa75fe4c2a54b2e  Compiled from "TestClass.java"public class com.hx.TestClass extends com.hx.vo.BaseParam implements java.lang.Runnable  minor version: 0  major version: 51  flags: ACC_PUBLIC, ACC_SUPERConstant pool:   #1 = Class              #2             // com/hx/TestClass   #2 = Utf8               com/hx/TestClass   #3 = Class              #4             // com/hx/vo/BaseParam   #4 = Utf8               com/hx/vo/BaseParam   #5 = Class              #6             // java/lang/Runnable   #6 = Utf8               java/lang/Runnable   #7 = Utf8               name   #8 = Utf8               Ljava/lang/String;   #9 = Utf8               AGE  #10 = Utf8               I  #11 = Utf8               ConstantValue  #12 = Integer            18  #13 = Utf8               <init>  #14 = Utf8               ()V  #15 = Utf8               Code  #16 = Methodref          #3.#17         // com/hx/vo/BaseParam."<init>":()V  #17 = NameAndType        #13:#14        // "<init>":()V  #18 = Utf8               LineNumberTable  #19 = Utf8               LocalVariableTable  #20 = Utf8               this  #21 = Utf8               Lcom/hx/TestClass;  #22 = Utf8               run  #23 = String             #24            // MichaelKoo,18  #24 = Utf8               MichaelKoo,18  #25 = Fieldref           #1.#26         // com/hx/TestClass.name:Ljava/lang/String;  #26 = NameAndType        #7:#8          // name:Ljava/lang/String;  #27 = Methodref          #28.#30        // java/lang/Object.getClass:()Ljava/lang/Class;  #28 = Class              #29            // java/lang/Object  #29 = Utf8               java/lang/Object  #30 = NameAndType        #31:#32        // getClass:()Ljava/lang/Class;  #31 = Utf8               getClass  #32 = Utf8               ()Ljava/lang/Class;  #33 = Methodref          #34.#36        // com/hx/CsdnUtil.getContent:(Ljava/lang/Class;)Ljava/lang/String;  #34 = Class              #35            // com/hx/CsdnUtil  #35 = Utf8               com/hx/CsdnUtil  #36 = NameAndType        #37:#38        // getContent:(Ljava/lang/Class;)Ljava/lang/String;  #37 = Utf8               getContent  #38 = Utf8               (Ljava/lang/Class;)Ljava/lang/String;  #39 = Methodref          #40.#42        // java/lang/Exception.printStackTrace:()V  #40 = Class              #41            // java/lang/Exception  #41 = Utf8               java/lang/Exception  #42 = NameAndType        #43:#14        // printStackTrace:()V  #43 = Utf8               printStackTrace  #44 = Utf8               e  #45 = Utf8               Ljava/lang/Exception;  #46 = Utf8               StackMapTable  #47 = Utf8               SourceFile  #48 = Utf8               TestClass.java{  public com.hx.TestClass();    descriptor: ()V    flags: ACC_PUBLIC    Code:      stack=1, locals=1, args_size=1         0: aload_0         1: invokespecial #16                 // Method com/hx/vo/BaseParam."<init>":()V         4: return      LineNumberTable:        line 13: 0      LocalVariableTable:        Start  Length  Slot  Name   Signature            0       5     0  this   Lcom/hx/TestClass;  public void run();    descriptor: ()V    flags: ACC_PUBLIC    Code:      stack=2, locals=2, args_size=1         0: aload_0         1: ldc           #23                 // String MichaelKoo,18         3: putfield      #25                 // Field name:Ljava/lang/String;         6: aload_0         7: invokevirtual #27                 // Method java/lang/Object.getClass:()Ljava/lang/Class;        10: invokestatic  #33                 // Method com/hx/CsdnUtil.getContent:(Ljava/lang/Class;)Ljava/lang/String;        13: pop        14: goto          22        17: astore_1        18: aload_1        19: invokevirtual #39                 // Method java/lang/Exception.printStackTrace:()V        22: return      Exception table:         from    to  target type             6    14    17   Class java/lang/Exception      LineNumberTable:        line 18: 0        line 21: 6        line 22: 14        line 23: 18        line 25: 22      LocalVariableTable:        Start  Length  Slot  Name   Signature            0      23     0  this   Lcom/hx/TestClass;           18       4     1     e   Ljava/lang/Exception;      StackMapTable: number_of_entries = 2        frame_type = 81 /* same_locals_1_stack_item */          stack = [ class java/lang/Exception ]        frame_type = 4 /* same */}SourceFile: "TestClass.java"

其中有几个需要关注的节点：
- 类的信息，在关键字’Constant pool:’之前就属于类的基本信息，从中可以看出，类的修饰符(public )、类的完整名称(com.hx.TestClass)、类的父类完整名称(com.hx.vo.BaseParam)、类的接口信息(java.lang.Runnable)
- 常量池，在关键字’Constant pool:’之后，第一个’{‘之前的内容就属于常量池信息，
- 在’{‘之后，’}’之前就是类的方法信息，从中可以看出:方法的修饰符、方法的返回值类型、方法名称、方法的异常表、方法的局部变量表等信息