Java各个方法调用浅析

JVM方法调用的指令有以下五种:

invokestatic 调用类方法(静态绑定,速度快)

invokevirtual 调用实例方法(动态绑定)

invokespecial 调用构造方法,私有方法及super关键字方法(静态绑定,速度快)

invokeinterface 调用接口方法(动态绑定)

invokedynamic JDK7引入的支持动态语言的方法调用

让我们通过字节码实例来分析一下(例子是使用Java8编译的)

package com.zou.basic.class1;

import java.util.Arrays;

public class A implements IA {

    public A() {

    }

    public static int add(int a, int b) {
        return a + b;
    }

    public int minus(int a, int b) {
        return a - b;
    }

    public int plus(int a, int b) {
        return a * b;
    }

    private int power(int a) {
        int hash = super.hashCode();
        return a * a;
    }

    public static void main(String[] args) {
        // invokestatic
        A.add(10, 20);

        // invokespecial
        A a = new A();
        a.power(10);
        // invokevirtual
        a.minus(20, 10);
        // invokeinterface
        IA a1 = (IA) a;
        a1.plus(10, 20);
        // invokedynamic
        Arrays.asList(1, 2, 3, 4).stream().forEach(n -> {
            System.out.println(n);
        });
    }

}

interface IA {

    int plus(int a, int b);

}

相应的main方法的字节码如下

// class version 52.0 (52)
// access flags 0x21
public class com/zou/basic/class1/A implements com/zou/basic/class1/IA  {

  // compiled from: A.java
  // access flags 0x19
  public final static INNERCLASS java/lang/invoke/MethodHandles$Lookup java/lang/invoke/MethodHandles Lookup

  // access flags 0x1
  public ()V
   L0
    LINENUMBER 9 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object. ()V
   L1
    LINENUMBER 11 L1
    RETURN
   L2
    LOCALVARIABLE this Lcom/zou/basic/class1/A; L0 L2 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x9
  public static add(II)I
   L0
    LINENUMBER 14 L0
    ILOAD 0
    ILOAD 1
    IADD
    IRETURN
   L1
    LOCALVARIABLE a I L0 L1 0
    LOCALVARIABLE b I L0 L1 1
    MAXSTACK = 2
    MAXLOCALS = 2

  // access flags 0x1
  public minus(II)I
   L0
    LINENUMBER 18 L0
    ILOAD 1
    ILOAD 2
    ISUB
    IRETURN
   L1
    LOCALVARIABLE this Lcom/zou/basic/class1/A; L0 L1 0
    LOCALVARIABLE a I L0 L1 1
    LOCALVARIABLE b I L0 L1 2
    MAXSTACK = 2
    MAXLOCALS = 3

  // access flags 0x1
  public plus(II)I
   L0
    LINENUMBER 22 L0
    ILOAD 1
    ILOAD 2
    IMUL
    IRETURN
   L1
    LOCALVARIABLE this Lcom/zou/basic/class1/A; L0 L1 0
    LOCALVARIABLE a I L0 L1 1
    LOCALVARIABLE b I L0 L1 2
    MAXSTACK = 2
    MAXLOCALS = 3

  // access flags 0x2
  private power(I)I
   L0
    LINENUMBER 26 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.hashCode ()I
    ISTORE 2
   L1
    LINENUMBER 27 L1
    ILOAD 1
    ILOAD 1
    IMUL
    IRETURN
   L2
    LOCALVARIABLE this Lcom/zou/basic/class1/A; L0 L2 0
    LOCALVARIABLE a I L0 L2 1
    LOCALVARIABLE hash I L1 L2 2
    MAXSTACK = 2
    MAXLOCALS = 3

  // access flags 0x9
  public static main([Ljava/lang/String;)V
   L0
    LINENUMBER 32 L0
    BIPUSH 10
    BIPUSH 20
    INVOKESTATIC com/zou/basic/class1/A.add (II)I
    POP
   L1
    LINENUMBER 35 L1
    NEW com/zou/basic/class1/A
    DUP
    INVOKESPECIAL com/zou/basic/class1/A. ()V
    ASTORE 1
   L2
    LINENUMBER 36 L2
    ALOAD 1
    BIPUSH 10
    INVOKESPECIAL com/zou/basic/class1/A.power (I)I
    POP
   L3
    LINENUMBER 38 L3
    ALOAD 1
    BIPUSH 20
    BIPUSH 10
    INVOKEVIRTUAL com/zou/basic/class1/A.minus (II)I
    POP
   L4
    LINENUMBER 40 L4
    ALOAD 1
    ASTORE 2
   L5
    LINENUMBER 41 L5
    ALOAD 2
    BIPUSH 10
    BIPUSH 20
    INVOKEINTERFACE com/zou/basic/class1/IA.plus (II)I
    POP
   L6
    LINENUMBER 43 L6
    ICONST_4
    ANEWARRAY java/lang/Integer
    DUP
    ICONST_0
    ICONST_1
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    AASTORE
    DUP
    ICONST_1
    ICONST_2
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    AASTORE
    DUP
    ICONST_2
    ICONST_3
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    AASTORE
    DUP
    ICONST_3
    ICONST_4
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    AASTORE
    INVOKESTATIC java/util/Arrays.asList ([Ljava/lang/Object;)Ljava/util/List;
    INVOKEINTERFACE java/util/List.stream ()Ljava/util/stream/Stream;
    INVOKEDYNAMIC accept()Ljava/util/function/Consumer; [
      // handle kind 0x6 : INVOKESTATIC
      java/lang/invoke/LambdaMetafactory.metafactory(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
      // arguments:
      (Ljava/lang/Object;)V, 
      // handle kind 0x6 : INVOKESTATIC
      com/zou/basic/class1/A.lambda$main$0(Ljava/lang/Integer;)V, 
      (Ljava/lang/Integer;)V
    ]
    INVOKEINTERFACE java/util/stream/Stream.forEach (Ljava/util/function/Consumer;)V
   L7
    LINENUMBER 46 L7
    RETURN
   L8
    LOCALVARIABLE args [Ljava/lang/String; L0 L8 0
    LOCALVARIABLE a Lcom/zou/basic/class1/A; L2 L8 1
    LOCALVARIABLE a1 Lcom/zou/basic/class1/IA; L5 L8 2
    MAXSTACK = 4
    MAXLOCALS = 3

  // access flags 0x100A
  private static synthetic lambda$main$0(Ljava/lang/Integer;)V
   L0
    LINENUMBER 44 L0
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 0
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/Object;)V
   L1
    LINENUMBER 45 L1
    RETURN
   L2
    LOCALVARIABLE n Ljava/lang/Integer; L0 L2 0
    MAXSTACK = 2
    MAXLOCALS = 1
}

暂时不支持行号,所以相应的字节码在那个位置请各位看官自行查找。

  • invokestatic 调用静态方法add的时候是字节码中是
L2
    LINENUMBER 36 L2
    ALOAD 1
    BIPUSH 10
    INVOKESPECIAL com/zou/basic/class1/A.power (I)I
    POP
  • invokevirtual 调用实例方法minus的时候的字节码:
L3
    LINENUMBER 38 L3
    ALOAD 1
    BIPUSH 20
    BIPUSH 10
    INVOKEVIRTUAL com/zou/basic/class1/A.minus (II)I
    POP
  • invokespecial 调用minus的时候字节码:
L2
    LINENUMBER 26 L2
    NEW com/zou/basic/class1/A
    DUP
    INVOKESPECIAL com/zou/basic/class1/A. ()V
    ASTORE 2

调用power方法的时候的字节码

L2
    LINENUMBER 36 L2
    ALOAD 1
    BIPUSH 10
    INVOKESPECIAL com/zou/basic/class1/A.power (I)I
    POP

power方法中调用Object.hashCode方法的字节码INVOKESPECIAL java/lang/Object.hashCode ()I
从以上三个调用可以看出来调用构造方法、调用private方法及super调用的时候都是使用invokespecial指令

  • invokeinterface 调用接口方法plus的时候的字节码:INVOKEINTERFACE com/zou/basic/class1/IA.plus (II)I,注意如果不强转成接口IA去调用plus方法,那么其实使用的还是invokevirtual
  • invokedynamic Java7引入的字节码指令,是为JVM上的动态语言如Groovy,Scala, JRuby等,同时也是java8 lamda表达式的基础,有了这个指令,JVM才能更高效的产生动态的类,可以看到例子中lamda表达式其实是通过invokedynamic指令由LambdaMetafactory.metafactory动态生成了一个java.util.function.Consumer的类,invokedynamic并不是用来调用方法的,他是用来动态生成动态类的,不如lamda表达式背后其实就是一个类。
 INVOKEDYNAMIC accept()Ljava/util/function/Consumer; [
      // handle kind 0x6 : INVOKESTATIC
      java/lang/invoke/LambdaMetafactory.metafactory(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
      // arguments:
      (Ljava/lang/Object;)V, 
      // handle kind 0x6 : INVOKESTATIC
      com/zou/basic/class1/A.lambda$main$0(Ljava/lang/Integer;)V, 
      (Ljava/lang/Integer;)V
    ]

参考文献:
http://stackoverflow.com/questions/30002380/why-are-java-8-lambdas-invoked-using-invokedynamic

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