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