上级目录

概览

从 Instruction 的角度来说，与 return 相关的 opcode 有 6 个，内容如下：

opcode mnemonic symbol opcode mnemonic symbol opcode mnemonic symbol
172 ireturn 174 freturn 176 areturn
173 lreturn 175 dreturn 177 return

从 ASM 的角度来说，这些 opcode 是通过 MethodVisitor.visitInsn(int opcode) 方法来调用的。

return void type

从 Java 语言的视角，有一个 HelloWorld 类，代码如下：

public class HelloWorld {
    public void test() {
        // do nothing
    }
}

从 Instruction 的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld
Compiled from "HelloWorld.java"
public class sample.HelloWorld {
...
  public void test();
    Code:
       0: return
}

从 ASM 的视角来看，方法体对应的内容如下：

methodVisitor.visitCode();
methodVisitor.visitInsn(RETURN);
methodVisitor.visitMaxs(0, 1);
methodVisitor.visitEnd();

从 Frame 的视角来看，local variable 和 operand stack 的变化：

                               // {this} | {}
0000: return                   // {} | {}

从 JVM 规范的角度来看，Operand Stack 的变化如下：

... →

[empty]

The current method must have return type void.

  • If no exception is thrown, any values on the operand stack of the current frame are discarded.
  • The interpreter then returns control to the invoker of the method, reinstating the frame of the invoker.

return primitive type

ireturn

从 Java 语言的视角，有一个 HelloWorld 类，代码如下：

public class HelloWorld {
    public int test() {
        return 0;
    }
}

从 Instruction 的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld
Compiled from "HelloWorld.java"
public class sample.HelloWorld {
...
  public int test();
    Code:
       0: iconst_0
       1: ireturn
}

从 ASM 的视角来看，方法体对应的内容如下：

methodVisitor.visitCode();
methodVisitor.visitInsn(ICONST_0);
methodVisitor.visitInsn(IRETURN);
methodVisitor.visitMaxs(1, 1);
methodVisitor.visitEnd();

从 Frame 的视角来看，local variable 和 operand stack 的变化：

                               // {this} | {}
0000: iconst_0                 // {this} | {int}
0001: ireturn                  // {} | {}

从 JVM 规范的角度来看，Operand Stack 的变化如下：

..., value →

[empty]
  • The current method must have return type boolean, byte, short, char, or int. The value must be of type int.
  • If no exception is thrown, value is popped from the operand stack of the current frame and pushed onto the operand stack of the frame of the invoker.
  • Any other values on the operand stack of the current method are discarded.
  • The interpreter then returns control to the invoker of the method, reinstating the frame of the invoker.

freturn

从 Java 语言的视角，有一个 HelloWorld 类，代码如下：

public class HelloWorld {
    public float test() {
        return 0;
    }
}

从 Instruction 的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld
Compiled from "HelloWorld.java"
public class sample.HelloWorld {
...
  public float test();
    Code:
       0: fconst_0
       1: freturn
}

从 ASM 的视角来看，方法体对应的内容如下：

methodVisitor.visitCode();
methodVisitor.visitInsn(FCONST_0);
methodVisitor.visitInsn(FRETURN);
methodVisitor.visitMaxs(1, 1);
methodVisitor.visitEnd();

从 Frame 的视角来看，local variable 和 operand stack 的变化：

                               // {this} | {}
0000: fconst_0                 // {this} | {float}
0001: freturn                  // {} | {}

从 JVM 规范的角度来看，Operand Stack 的变化如下：

..., value →

[empty]
  • The current method must have return type float. The value must be of type float.
  • If no exception is thrown, value is popped from the operand stack of the current frame. The value is pushed onto the operand stack of the frame of the invoker.
  • Any other values on the operand stack of the current method are discarded.
  • The interpreter then returns control to the invoker of the method, reinstating the frame of the invoker.

lreturn

这里需要注意的就是 long 类型在 local variable 和 operand stack 当中占用 2 个 slot 的位置。

从 Java 语言的视角，有一个 HelloWorld 类，代码如下：

public class HelloWorld {
    public long test() {
        return 0;
    }
}

从 Instruction 的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld
Compiled from "HelloWorld.java"
public class sample.HelloWorld {
...
  public long test();
    Code:
       0: lconst_0
       1: lreturn
}

从 ASM 的视角来看，方法体对应的内容如下：

methodVisitor.visitCode();
methodVisitor.visitInsn(LCONST_0);
methodVisitor.visitInsn(LRETURN);
methodVisitor.visitMaxs(2, 1);
methodVisitor.visitEnd();

从 Frame 的视角来看，local variable 和 operand stack 的变化：

                               // {this} | {}
0000: lconst_0                 // {this} | {long, top}
0001: lreturn                  // {} | {}

从 JVM 规范的角度来看，Operand Stack 的变化如下：

..., value →

[empty]
  • The current method must have return type long. The value must be of type long.
  • If no exception is thrown, value is popped from the operand stack of the current frame and pushed onto the operand stack of the frame of the invoker.
  • Any other values on the operand stack of the current method are discarded.
  • The interpreter then returns control to the invoker of the method, reinstating the frame of the invoker.

dreturn

从 Java 语言的视角，有一个 HelloWorld 类，代码如下：

public class HelloWorld {
    public double test() {
        return 0;
    }
}

从 Instruction 的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld
Compiled from "HelloWorld.java"
public class sample.HelloWorld {
...
  public double test();
    Code:
       0: dconst_0
       1: dreturn
}

从 ASM 的视角来看，方法体对应的内容如下：

methodVisitor.visitCode();
methodVisitor.visitInsn(DCONST_0);
methodVisitor.visitInsn(DRETURN);
methodVisitor.visitMaxs(2, 1);
methodVisitor.visitEnd();

从 Frame 的视角来看，local variable 和 operand stack 的变化：

                               // {this} | {}
0000: dconst_0                 // {this} | {double, top}
0001: dreturn                  // {} | {}

从 JVM 规范的角度来看，Operand Stack 的变化如下：

..., value →

[empty]
  • The current method must have return type double. The value must be of type double.
  • If no exception is thrown, value is popped from the operand stack of the current frame. The value is pushed onto the operand stack of the frame of the invoker.
  • Any other values on the operand stack of the current method are discarded.
  • The interpreter then returns control to the invoker of the method, reinstating the frame of the invoker.

return reference type

从 Java 语言的视角，有一个 HelloWorld 类，代码如下：

public class HelloWorld {
    public Object test() {
        return null;
    }
}

从 Instruction 的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld
Compiled from "HelloWorld.java"
public class sample.HelloWorld {
...
  public java.lang.Object test();
    Code:
       0: aconst_null
       1: areturn
}

从 ASM 的视角来看，方法体对应的内容如下：

methodVisitor.visitCode();
methodVisitor.visitInsn(ACONST_NULL);
methodVisitor.visitInsn(ARETURN);
methodVisitor.visitMaxs(1, 1);
methodVisitor.visitEnd();

从 Frame 的视角来看，local variable 和 operand stack 的变化：

                               // {this} | {}
0000: aconst_null              // {this} | {null}
0001: areturn                  // {} | {}

从 JVM 规范的角度来看，Operand Stack 的变化如下：

..., objectref →

[empty]
  • The objectref must be of type reference and must refer to an object of a type that is assignment compatible with the type represented by the return descriptor of the current method.
  • If no exception is thrown, objectref is popped from the operand stack of the current frame and pushed onto the operand stack of the frame of the invoker.
  • Any other values on the operand stack of the current method are discarded.
  • The interpreter then reinstates the frame of the invoker and returns control to the invoker.