上级目录

概览

从 Instruction 的角度来说，与 field 相关的 opcode 有 4 个，内容如下：

opcode mnemonic symbol opcode mnemonic symbol opcode mnemonic symbol opcode mnemonic symbol
178 getstatic 179 putstatic 180 getfield 181 putfield

从 ASM 的角度来说，这些 opcode 与 MethodVisitor.visitXxxInsn() 方法对应关系如下：

  • MethodVisitor.visitFieldInsn(): getstatic, putstatic, getfield, putfield.

non-static field

getfield

从 Java 语言的视角，有一个 HelloWorld 类，代码如下：

public class HelloWorld {
    public int value;

    public void test() {
        int i = this.value;
    }
}

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 value;
...
  public void test();
    Code:
       0: aload_0
       1: getfield      #2                  // Field value:I
       4: istore_1
       5: return
}

从 ASM 的视角来看，方法体对应的内容如下：

methodVisitor.visitCode();
methodVisitor.visitVarInsn(ALOAD, 0);
methodVisitor.visitFieldInsn(GETFIELD, "sample/HelloWorld", "value", "I");
methodVisitor.visitVarInsn(ISTORE, 1);
methodVisitor.visitInsn(RETURN);
methodVisitor.visitMaxs(1, 2);
methodVisitor.visitEnd();

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

                               // {this} | {}
0000: aload_0                  // {this} | {this}
0001: getfield        #2       // {this} | {int}
0004: istore_1                 // {this, int} | {}
0005: return                   // {} | {}

从 JVM 规范的角度来看，getfield 指令对应的 Operand Stack 的变化如下：

..., objectref →

..., value

The objectref, which must be of type reference, is popped from the operand stack. The value of the referenced field in objectref is fetched and pushed onto the operand stack.

putfield

从 Java 语言的视角，有一个 HelloWorld 类，代码如下：

public class HelloWorld {
    public int value;

    public void test() {
        this.value = 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 value;
...

  public void test();
    Code:
       0: aload_0
       1: iconst_0
       2: putfield      #2                  // Field value:I
       5: return
}

从 ASM 的视角来看，方法体对应的内容如下：

methodVisitor.visitCode();
methodVisitor.visitVarInsn(ALOAD, 0);
methodVisitor.visitInsn(ICONST_0);
methodVisitor.visitFieldInsn(PUTFIELD, "sample/HelloWorld", "value", "I");
methodVisitor.visitInsn(RETURN);
methodVisitor.visitMaxs(2, 1);
methodVisitor.visitEnd();

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

                               // {this} | {}
0000: aload_0                  // {this} | {this}
0001: iconst_0                 // {this} | {this, int}
0002: putfield        #2       // {this} | {}
0005: return                   // {} | {}

从 JVM 规范的角度来看，putfield 指令对应的 Operand Stack 的变化如下：

..., objectref, value →

...

The value and objectref are popped from the operand stack. The objectref must be of type reference. The value undergoes value set conversion, resulting in value', and the referenced field in objectref is set to value'.

  • If the field descriptor type is boolean, byte, char, short, or int, then the value must be an int.
  • If the field descriptor type is float, long, or double, then the value must be a float, long, or double, respectively.
  • If the field descriptor type is a reference type, then the value must be of a type that is assignment compatible with the field descriptor type.
  • If the field is final, it must be declared in the current class, and the instruction must occur in an instance initialization method (<init>) of the current class.

static field

getstatic

从 Java 语言的视角，有一个 HelloWorld 类，代码如下：

public class HelloWorld {
    public static int staticValue;

    public void test() {
        int i = HelloWorld.staticValue;
    }
}

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 static int staticValue;
...

  public void test();
    Code:
       0: getstatic     #2                  // Field staticValue:I
       3: istore_1
       4: return
}

从 ASM 的视角来看，方法体对应的内容如下：

methodVisitor.visitCode();
methodVisitor.visitFieldInsn(GETSTATIC, "sample/HelloWorld", "staticValue", "I");
methodVisitor.visitVarInsn(ISTORE, 1);
methodVisitor.visitInsn(RETURN);
methodVisitor.visitMaxs(1, 2);
methodVisitor.visitEnd();

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

                               // {this} | {}
0000: getstatic       #2       // {this} | {int}
0003: istore_1                 // {this, int} | {}
0004: return                   // {} | {}

从 JVM 规范的角度来看，getstatic 指令对应的 Operand Stack 的变化如下：

..., →

..., value

The value of the class or interface field is fetched and pushed onto the operand stack.

putstatic

从 Java 语言的视角，有一个 HelloWorld 类，代码如下：

public class HelloWorld {
    public static int staticValue;

    public void test() {
        HelloWorld.staticValue = 1;
    }
}

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 static int staticValue;
...

  public void test();
    Code:
       0: iconst_1
       1: putstatic     #2                  // Field staticValue:I
       4: return
}

从 ASM 的视角来看，方法体对应的内容如下：

methodVisitor.visitCode();
methodVisitor.visitInsn(ICONST_1);
methodVisitor.visitFieldInsn(PUTSTATIC, "sample/HelloWorld", "staticValue", "I");
methodVisitor.visitInsn(RETURN);
methodVisitor.visitMaxs(1, 1);
methodVisitor.visitEnd();

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

                               // {this} | {}
0000: iconst_1                 // {this} | {int}
0001: putstatic       #2       // {this} | {}
0004: return                   // {} | {}

从 JVM 规范的角度来看，putstatic 指令对应的 Operand Stack 的变化如下：

..., value →

...

The value is popped from the operand stack and undergoes value set conversion, resulting in value'. The class field is set to value'.

The type of a value stored by a putstatic instruction must be compatible with the descriptor of the referenced field.

  • If the field descriptor type is boolean, byte, char, short, or int, then the value must be an int.
  • If the field descriptor type is float, long, or double, then the value must be a float, long, or double, respectively.
  • If the field descriptor type is a reference type, then the value must be of a type that is assignment compatible with the field descriptor type.
  • If the field is final, it must be declared in the current class, and the instruction must occur in the <clinit> method of the current class.