Java class 文件结构分析-优快云博客

1. 最简单的一段java代码

public class Test1 {
	public int getNum(int i) {
		return i + 1;
	}
}

对编译之后的class文件，用javap分析该文件，得出的伪机器码

下面对机器码分析

public class Test1 extends java.lang.Object
  SourceFile: "Test1.java"
  minor version: 0
  major version: 50
//这是常量池，进入JVM之后，会被储存在PermArea里
  Constant pool:
const #1 = class        #2;     //  Test1
const #2 = Asciz        Test1;
const #3 = class        #4;     //  java/lang/Object
const #4 = Asciz        java/lang/Object;
const #5 = Asciz        <init>;
const #6 = Asciz        ()V;
const #7 = Asciz        Code;
const #8 = Method       #3.#9;  //  java/lang/Object."<init>":()V
const #9 = NameAndType  #5:#6;//  "<init>":()V
const #10 = Asciz       LineNumberTable;
const #11 = Asciz       LocalVariableTable;
const #12 = Asciz       this;
const #13 = Asciz       LTest1;;
const #14 = Asciz       getNum;
const #15 = Asciz       (I)I;
const #16 = Asciz       i;
const #17 = Asciz       I;
const #18 = Asciz       SourceFile;
const #19 = Asciz       Test1.java;

{
//构造函数
//线程stack只有一层
//有一个变量 就是this变量
public Test1();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #8; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 2: 0

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      5      0    this       LTest1;

//自定义方法，stack两层
public int getNum(int);
  Code:
   Stack=2, Locals=2, Args_size=2
//以下是逻辑代码的解释
//首先加载变量
//然后赋值1
//然后加
//然后返回
   0:   iload_1
   1:   iconst_1
   2:   iadd
   3:   ireturn
  LineNumberTable:
   line 4: 0

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      4      0    this       LTest1;
   0      4      1    i       I


}

2. 代码稍微复杂一点

加上异常处理和对象

import java.io.File;

public class Test1 {
	public int getNum(int i) {
		try {
			new File("xxx.xxx");
			for (int k = 0; k < i; i++) {
				i = i + 1;
			}
			return i;
		} catch (Exception e) {
			return 1;
		}finally{
		
		}
	}
}

Javap分析之后的机器码

public class Test1 extends java.lang.Object
  SourceFile: "Test1.java"
  minor version: 0
  major version: 50
//常量池，不废话
  Constant pool:
const #1 = class        #2;     //  Test1
const #2 = Asciz        Test1;
const #3 = class        #4;     //  java/lang/Object
const #4 = Asciz        java/lang/Object;
const #5 = Asciz        <init>;
const #6 = Asciz        ()V;
const #7 = Asciz        Code;
const #8 = Method       #3.#9;  //  java/lang/Object."<init>":()V
const #9 = NameAndType  #5:#6;//  "<init>":()V
const #10 = Asciz       LineNumberTable;
const #11 = Asciz       LocalVariableTable;
const #12 = Asciz       this;
const #13 = Asciz       LTest1;;
const #14 = Asciz       getNum;
const #15 = Asciz       (I)I;
const #16 = class       #17;    //  java/io/File
const #17 = Asciz       java/io/File;
const #18 = String      #19;    //  xxx.xxx
const #19 = Asciz       xxx.xxx;
const #20 = Method      #16.#21;        //  java/io/File."<init>":(Ljava/lang/St
ring;)V
const #21 = NameAndType #5:#22;//  "<init>":(Ljava/lang/String;)V
const #22 = Asciz       (Ljava/lang/String;)V;
const #23 = class       #24;    //  java/lang/Exception
const #24 = Asciz       java/lang/Exception;
const #25 = Asciz       i;
const #26 = Asciz       I;
const #27 = Asciz       k;
const #28 = Asciz       e;
const #29 = Asciz       Ljava/lang/Exception;;
const #30 = Asciz       StackMapTable;
const #31 = Asciz       SourceFile;
const #32 = Asciz       Test1.java;

{
//构造函数 不废话
public Test1();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #8; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 3: 0

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      5      0    this       LTest1;


public int getNum(int);
  Code:
   Stack=2, Locals=3, Args_size=2
//new ？ jvm自带的机器码？
   0:   new     #16; //class java/io/File
   3:   ldc     #18; //String xxx.xxx
   5:   invokespecial   #20; //Method java/io/File."<init>":(Ljava/lang/String;)
V
   8:   iconst_0
   9:   istore_2
   10:  goto    19
// i自加1
   13:  iinc    1, 1
//k自加1
   16:  iinc    1, 1
//加载i变量
   19:  iload_2
//加载k变量
   20:  iload_1
//如果k<I 接着循环
   21:  if_icmplt       13
//加载变量I 并且返回
   24:  iload_1
   25:  ireturn
//返回常量1
   26:  astore_2
   27:  iconst_1
   28:  ireturn
//异常表
  Exception table:
//25行到26行之间捕获异常
   from   to  target type
     0    25    26   Class java/lang/Exception

  LineNumberTable:
   line 6: 0
   line 7: 8
   line 8: 13
   line 7: 16
   line 10: 24
   line 11: 26
   line 12: 27

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      29      0    this       LTest1;
   0      29      1    i       I
   10      14      2    k       I
   27      2      2    e       Ljava/lang/Exception;

  StackMapTable: number_of_entries = 3
   frame_type = 252 /* append */
     offset_delta = 13
     locals = [ int ]
   frame_type = 5 /* same */
   frame_type = 255 /* full_frame */
     offset_delta = 6
     locals = [ class Test1, int ]
     stack = [ class java/lang/Exception ]


}

以上疯狂的加班中