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;)Vflags: ACC_PUBLICCode:stack=1, locals=4, args_size=30: ldc           #2                  // String A2: astore_33: returnLineNumberTable:line 13: 0line 14: 3LocalVariableTable:Start  Length  Slot  Name   Signature0       4     0  this   Lcom/ghgcn/jvm/chapter02/MyJvmTest;0       4     1     d   I0       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=0if(true){//locals=1String a ="a";}//locals=0if(true){//locals =1String b = "b";}//locals =0}
}

class

public static void foo();descriptor: ()Vflags: ACC_PUBLIC, ACC_STATICCode:stack=1, locals=1, args_size=00: ldc           #2                  // String a2: astore_03: ldc           #3                  // String b5: astore_06: returnLineNumberTable:line 30: 0line 33: 3line 36: 6LocalVariableTable: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: ()Vflags: ACC_PUBLICCode:stack=4, locals=1, args_size=10: aload_01: iconst_12: iconst_23: iconst_34: invokespecial #2                  // Method bar:(III)V7: returnLineNumberTable:line 14: 0line 16: 7LocalVariableTable:Start  Length  Slot  Name   Signature0       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: ()Vflags: ACC_PUBLICCode:stack=4, locals=1, args_size=10: aload_01: iconst_12: iconst_23: iconst_34: invokespecial #2                  // Method bar:(III)V7: aload_08: iconst_19: invokespecial #3                  // Method bar:(I)V12: returnLineNumberTable:line 14: 0line 15: 7line 17: 12LocalVariableTable:Start  Length  Slot  Name   Signature0      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: ()Vflags: ACC_PUBLICCode:stack=6, locals=1, args_size=10: aload_01: iconst_12: iconst_23: iconst_34: invokespecial #2                  // Method bar:(III)V7: aload_08: iconst_19: invokespecial #3                  // Method bar:(I)V12: aload_013: iconst_114: iconst_215: iconst_316: iconst_417: iconst_518: invokespecial #4                  // Method bar:(IIIII)V21: returnLineNumberTable:line 14: 0line 15: 7line 16: 12line 18: 21LocalVariableTable:Start  Length  Slot  Name   Signature0      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