JVM学习笔记（五）：根据字节码指令介绍方法执行流程

在上一篇博客中介绍了《Class文件结构》，其中就提到了一个例子，下面我们依然根据该例子的字节码来对方法的执行流程进行讲解。

     java类源码如下：

 

public class TestClass{
     private int num;

     public int inc(){
         for(int i=0; i<10; i++){
            num = i;
        }
         return num;
    }

     public static void main(String[] args){
         new TestClass().inc();
    }

}

public class TestClass{
	private int num;
	
	public int inc(){
		for(int i=0; i<10; i++){
			num = i;
		}
		return num;
	}
	
	public static void main(String[] args){
		new TestClass().inc();
	}
	
}

使用javap -verbose命令反编译后，输出常量表和字节码如下：

 

D:\JVM>javap -verbose TestClass
 Compiled from "TestClass.java"
public class TestClass extends java.lang.Object
   SourceFile: "TestClass.java"
   minor version: 0
   major version: 50
  Constant pool:
 const #1 = Method       #6.#20; //  java/lang/Object."<init>":()V
 const #2 = Field        #3.#21; //  TestClass.num:I
 const #3 = class        #22;    //  TestClass
 const #4 = Method       #3.#20; //  TestClass."<init>":()V
 const #5 = Method       #3.#23; //  TestClass.inc:()I
 const #6 = class        #24;    //  java/lang/Object
 const #7 = Asciz        num;
 const #8 = Asciz        I;
 const #9 = Asciz        <init>;
 const #10 = Asciz       ()V;
 const #11 = Asciz       Code;
 const #12 = Asciz       LineNumberTable;
 const #13 = Asciz       inc;
 const #14 = Asciz       ()I;
 const #15 = Asciz       StackMapTable;
 const #16 = Asciz       main;
 const #17 = Asciz       ([Ljava/lang/String;)V;
 const #18 = Asciz       SourceFile;
 const #19 = Asciz       TestClass.java;
 const #20 = NameAndType #9:#10;//  "<init>":()V
 const #21 = NameAndType #7:#8;//  num:I
 const #22 = Asciz       TestClass;
 const #23 = NameAndType #13:#14;//  inc:()I
 const #24 = Asciz       java/lang/Object;

{
public TestClass();
  Code:
    Stack=1, Locals=1, Args_size=1
    0:   aload_0
    1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
    4:   return
  LineNumberTable:
    line 1: 0


public int inc();
  Code:
    Stack=2, Locals=2, Args_size=1
    0:   iconst_0        //定义一个常量0，放入操作数栈
    1:   istore_1        //把该常量弹出栈顶存入到局部变量表
    2:   iload_1         //把该局部变量放入操作数栈
    3:   bipush  10      //把常量10放入操作数栈
    5:   if_icmpge   19  //把i和10进行比较
    8:   aload_0         //加载局部变量表index为0的变量放入操作数栈
    9:   iload_1         //加载局部变量表index为1的变量放入操作数栈
    10:  putfield        #2; //Field num:I   //把i的值赋给num字段
    13:  iinc    1, 1    //局部变量i自增1
    16:  goto    2       //跳转到第2行
    19:  aload_0         //加载局部变量表index为0的变量放入操作数栈
    20:  getfield        #2; //Field num:I   //获取字段num的值
    23:  ireturn         //返回
  LineNumberTable:
    line 5: 0
    line 6: 8
    line 5: 13
    line 8: 19

   StackMapTable: number_of_entries = 2
    frame_type = 252 /* append */
      offset_delta = 2
     locals = [ int ]
    frame_type = 250 /* chop */
      offset_delta = 16


public static void main(java.lang.String[]);
  Code:
    Stack=2, Locals=1, Args_size=1
    0:   new     #3; //class TestClass
    3:   dup
    4:   invokespecial   #4; //Method "<init>":()V //调用实例初始化方法
    7:   invokevirtual   #5; //Method inc:()I        //调用普通方法inc()
    10:  pop
    11:  return
  LineNumberTable:
    line 12: 0
    line 13: 11


}

D:\JVM>javap -verbose TestClass
Compiled from "TestClass.java"
public class TestClass extends java.lang.Object
  SourceFile: "TestClass.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method       #6.#20; //  java/lang/Object."<init>":()V
const #2 = Field        #3.#21; //  TestClass.num:I
const #3 = class        #22;    //  TestClass
const #4 = Method       #3.#20; //  TestClass."<init>":()V
const #5 = Method       #3.#23; //  TestClass.inc:()I
const #6 = class        #24;    //  java/lang/Object
const #7 = Asciz        num;
const #8 = Asciz        I;
const #9 = Asciz        <init>;
const #10 = Asciz       ()V;
const #11 = Asciz       Code;
const #12 = Asciz       LineNumberTable;
const #13 = Asciz       inc;
const #14 = Asciz       ()I;
const #15 = Asciz       StackMapTable;
const #16 = Asciz       main;
const #17 = Asciz       ([Ljava/lang/String;)V;
const #18 = Asciz       SourceFile;
const #19 = Asciz       TestClass.java;
const #20 = NameAndType #9:#10;//  "<init>":()V
const #21 = NameAndType #7:#8;//  num:I
const #22 = Asciz       TestClass;
const #23 = NameAndType #13:#14;//  inc:()I
const #24 = Asciz       java/lang/Object;

{
public TestClass();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 1: 0


public int inc();
  Code:
   Stack=2, Locals=2, Args_size=1
   0:   iconst_0		//定义一个常量0，放入操作数栈
   1:   istore_1		//把该常量弹出栈顶存入到局部变量表
   2:   iload_1			//把该局部变量放入操作数栈
   3:   bipush  10		//把常量10放入操作数栈
   5:   if_icmpge   19	//把i和10进行比较
   8:   aload_0			//加载局部变量表index为0的变量放入操作数栈
   9:   iload_1			//加载局部变量表index为1的变量放入操作数栈
   10:  putfield     	#2; //Field num:I	//把i的值赋给num字段
   13:  iinc    1, 1	//局部变量i自增1
   16:  goto    2		//跳转到第2行
   19:  aload_0			//加载局部变量表index为0的变量放入操作数栈
   20:  getfield        #2; //Field num:I	//获取字段num的值
   23:  ireturn			//返回
  LineNumberTable:
   line 5: 0
   line 6: 8
   line 5: 13
   line 8: 19

  StackMapTable: number_of_entries = 2
   frame_type = 252 /* append */
     offset_delta = 2
     locals = [ int ]
   frame_type = 250 /* chop */
     offset_delta = 16


public static void main(java.lang.String[]);
  Code:
   Stack=2, Locals=1, Args_size=1
   0:   new     #3; //class TestClass
   3:   dup
   4:   invokespecial   #4; //Method "<init>":()V	//调用实例初始化方法
   7:   invokevirtual   #5; //Method inc:()I		//调用普通方法inc()
   10:  pop
   11:  return
  LineNumberTable:
   line 12: 0
   line 13: 11


}

 

方法的调用指令分为以下几种：

1. invokevirtual指令用于调用所有的虚方法。
2. invokeinterface指令用于调用接口方法，它会在运行时搜索一个实现了这个接口方法的对象，找出适合的方法进行调用。
3. invokespecial指令用于调用一些需要特殊处理的实例方法，包括实例构造器<init>化方法、私有方法和父类方法。
4. invokestatic指令用于调用静态方法（static方法）。