java class局部变量表

每个栈帧内部都包含一组称为局部变量表的变量列表,局部变量表的大小在编译期音就已经确定了,对应class文件中方法Code属性的max_locals字段,Java虚拟机会根据max_locals字段来分配方法执行过程中需要分配的最大局部变量表的容量

例一

public class MyJvmTest {

    public void foo(int d ,String name){

        String tmp ="A";
    }
}

class文件
javap -c -v -l MyJvmTest.class

public void foo(int, java.lang.String);
    descriptor: (ILjava/lang/String;)V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=4, args_size=3
         0: ldc           #2                  // String A
         2: astore_3
         3: return
      LineNumberTable:
        line 13: 0
        line 14: 3
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       4     0  this   Lcom/ghgcn/jvm/chapter02/MyJvmTest;
            0       4     1     d   I
            0       4     2  name   Ljava/lang/String;
            3       1     3   tmp   Ljava/lang/String;
}

locals= 4
第0个局部变量this Lcom/ghgcn/jvm/chapter02/MyJvmTest; 这个实例方法的对象的引用,静态方法没有这个this变量
第1个变量 d I int 类型
第2个变量 name Ljava/lang/String; Stringh引用类型
第3个变量 tmp Ljava/lang/String;

例二

public class MyJvmTest {

    public  static   void foo(){

        //locals=0
        if(true){
            //locals=1
            String a ="a";
        }
        //locals=0
        if(true){
        //locals =1
            String b = "b";
        }
        //locals =0
    }
}

class

public static void foo();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=1, args_size=0
         0: ldc           #2                  // String a
         2: astore_0
         3: ldc           #3                  // String b
         5: astore_0
         6: return
      LineNumberTable:
        line 30: 0
        line 33: 3
        line 36: 6
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
}

a和b共用同一个slot等一0的局部变量表位置,当double 和long类型时,这些变量会占用2个局部变量表的slot

操作数

public class MyJvmTest2 {
    

    public  void foo(){

        bar(1,2,3);

    }

    private void bar(int a, int b, int c) {

    }
}

 public void foo();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=4, locals=1, args_size=1
         0: aload_0
         1: iconst_1
         2: iconst_2
         3: iconst_3
         4: invokespecial #2                  // Method bar:(III)V
         7: return
      LineNumberTable:
        line 14: 0
        line 16: 7
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       8     0  this   Lcom/ghgcn/jvm/chapter02/MyJvmTest2;
}
```

stack表示4因为调用bar方法会将this,1,2,3这2个变量压栈,栈的深度为4,调用完后全部出栈.
java
```java
public class MyJvmTest3 {


    public  void foo(){

        bar(1,2,3);
        bar(1);

    }

    private void bar(int a) {

    }

    private void bar(int a, int b, int c) {

    }
}

class

 public void foo();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=4, locals=1, args_size=1
         0: aload_0
         1: iconst_1
         2: iconst_2
         3: iconst_3
         4: invokespecial #2                  // Method bar:(III)V
         7: aload_0
         8: iconst_1
         9: invokespecial #3                  // Method bar:(I)V
        12: return
      LineNumberTable:
        line 14: 0
        line 15: 7
        line 17: 12
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      13     0  this   Lcom/ghgcn/jvm/chapter02/MyJvmTest3;
}

java

public class MyJvmTest3 {


    public  void foo(){

        bar(1,2,3);
        bar(1);
        bar(1,2,3,4,5);

    }

    private void bar(int a, int b, int c, int d, int e) {
        
    }

    private void bar(int a) {

    }

    private void bar(int a, int b, int c) {

    }
}

class文件

public void foo();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=6, locals=1, args_size=1
         0: aload_0
         1: iconst_1
         2: iconst_2
         3: iconst_3
         4: invokespecial #2                  // Method bar:(III)V
         7: aload_0
         8: iconst_1
         9: invokespecial #3                  // Method bar:(I)V
        12: aload_0
        13: iconst_1
        14: iconst_2
        15: iconst_3
        16: iconst_4
        17: iconst_5
        18: invokespecial #4                  // Method bar:(IIIII)V
        21: return
      LineNumberTable:
        line 14: 0
        line 15: 7
        line 16: 12
        line 18: 21
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      22     0  this   Lcom/ghgcn/jvm/chapter02/MyJvmTest3;
}

stack=6,
遇到入栈的字节码指令,stack+=1或者stack+=2,根据不同的类型的指令类型,遇到出栈的字节码指令,stack则相应的减少,这个过程中stack的的最大值就是max_stack,也就是javap输出中的stack值