废话不说上源码
1.main方法
public static void main(String[] args) {
Function s = Integer::parseInt;
s.apply("10");
} ::具体看下一节
2.反编译一下
javap -v -p StreamTest结果如下
警告: 二进制文件StreamTest包含com.cui.subject.java.function.StreamTest
Classfile /E:/WorkSpace/subject/java/target/classes/com/cui/subject/java/function/StreamTest.class
Last modified 2020-6-15; size 1311 bytes
MD5 checksum 7b5bc92830d2402bc0d88bfe91926d3e
Compiled from "StreamTest.java"
public class com.cui.subject.java.function.StreamTest
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #6.#24 // java/lang/Object."":()V
#2 = InvokeDynamic #0:#30 // #0:apply:()Ljava/util/function/Function;
#3 = String #31 // 10
#4 = InterfaceMethodref #32.#33 // java/util/function/Function.apply:(Ljava/lang/Object;)Ljava/lang/Object;
#5 = Class #34 // com/cui/subject/java/function/StreamTest
#6 = Class #35 // java/lang/Object
#7 = Utf8
#8 = Utf8 ()V
#9 = Utf8 Code
#10 = Utf8 LineNumberTable
#11 = Utf8 LocalVariableTable
#12 = Utf8 this
#13 = Utf8 Lcom/cui/subject/java/function/StreamTest;
#14 = Utf8 main
#15 = Utf8 ([Ljava/lang/String;)V
#16 = Utf8 args
#17 = Utf8 [Ljava/lang/String;
#18 = Utf8 s
#19 = Utf8 Ljava/util/function/Function;
#20 = Utf8 LocalVariableTypeTable
#21 = Utf8 Ljava/util/function/Function;
#22 = Utf8 SourceFile
#23 = Utf8 StreamTest.java
#24 = NameAndType #7:#8 // "":()V
#25 = Utf8 BootstrapMethods
#26 = MethodHandle #6:#36 // invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#27 = MethodType #37 // (Ljava/lang/Object;)Ljava/lang/Object;
#28 = MethodHandle #6:#38 // invokestatic java/lang/Integer.parseInt:(Ljava/lang/String;)I
#29 = MethodType #39 // (Ljava/lang/String;)Ljava/lang/Integer;
#30 = NameAndType #40:#41 // apply:()Ljava/util/function/Function;
#31 = Utf8 10
#32 = Class #42 // java/util/function/Function
#33 = NameAndType #40:#37 // apply:(Ljava/lang/Object;)Ljava/lang/Object;
#34 = Utf8 com/cui/subject/java/function/StreamTest
#35 = Utf8 java/lang/Object
#36 = Methodref #43.#44 // java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#37 = Utf8 (Ljava/lang/Object;)Ljava/lang/Object;
#38 = Methodref #45.#46 // java/lang/Integer.parseInt:(Ljava/lang/String;)I
#39 = Utf8 (Ljava/lang/String;)Ljava/lang/Integer;
#40 = Utf8 apply
#41 = Utf8 ()Ljava/util/function/Function;
#42 = Utf8 java/util/function/Function
#43 = Class #47 // java/lang/invoke/LambdaMetafactory
#44 = NameAndType #48:#52 // metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#45 = Class #53 // java/lang/Integer
#46 = NameAndType #54:#55 // parseInt:(Ljava/lang/String;)I
#47 = Utf8 java/lang/invoke/LambdaMetafactory
#48 = Utf8 metafactory
#49 = Class #57 // java/lang/invoke/MethodHandles$Lookup
#50 = Utf8 Lookup
#51 = Utf8 InnerClasses
#52 = Utf8 (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#53 = Utf8 java/lang/Integer
#54 = Utf8 parseInt
#55 = Utf8 (Ljava/lang/String;)I
#56 = Class #58 // java/lang/invoke/MethodHandles
#57 = Utf8 java/lang/invoke/MethodHandles$Lookup
#58 = Utf8 java/lang/invoke/MethodHandles
{
public com.cui.subject.java.function.StreamTest();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 11: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/cui/subject/java/function/StreamTest;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: invokedynamic #2, 0 // InvokeDynamic #0:apply:()Ljava/util/function/Function;
5: astore_1
6: aload_1
7: ldc #3 // String 10
9: invokeinterface #4, 2 // InterfaceMethod java/util/function/Function.apply:(Ljava/lang/Object;)Ljava/lang/Object;
14: pop
15: return
LineNumberTable:
line 13: 0
line 14: 6
line 17: 15
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 args [Ljava/lang/String;
6 10 1 s Ljava/util/function/Function;
LocalVariableTypeTable:
Start Length Slot Name Signature
6 10 1 s Ljava/util/function/Function;
}
SourceFile: "StreamTest.java"
InnerClasses:
public static final #50= #49 of #56; //Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles
BootstrapMethods:
0: #26 invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
Method arguments:
#27 (Ljava/lang/Object;)Ljava/lang/Object;
#28 invokestatic java/lang/Integer.parseInt:(Ljava/lang/String;)I
#29 (Ljava/lang/String;)Ljava/lang/Integer;
3.invokedynamic
#2 = InvokeDynamic #0:#30 // #0:apply:()Ljava/util/function/Function;3.1 0在这里
BootstrapMethods:
0: #26 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;
Method arguments:
#27 (Ljava/lang/Object;)Ljava/lang/Object;
#28 invokestatic java/lang/Integer.parseInt:(Ljava/lang/String;)I
#29 (Ljava/lang/String;)Ljava/lang/Integer;BootstrapMethods
每一个invokedynamic指令的实例叫做一个动态调用点(dynamic call site), 动态调用点最开始是未链接状态(unlinked:表示还未指定该调用点要调用的方法), 动态调用点依靠引导方法来链接到具体的方法. 引导方法是由编译器生成, 在运行期当JVM第一次遇到invokedynamic指令时, 会调用引导方法来将invokedynamic指令所指定的名字(方法名,方法签名)和具体的执行代码(目标方法)链接起来, 引导方法的返回值永久的决定了调用点的行为.引导方法的返回值类型是java.lang.invoke.CallSite, 一个invokedynamic指令关联一个CallSite, 将所有的调用委托到CallSite当前的target(MethodHandle)
LambdaMetafactory.metafactory
public static CallSite metafactory(MethodHandles.Lookup caller,
String invokedName,
MethodType invokedType,
MethodType samMethodType,
MethodHandle implMethod,
MethodType instantiatedMethodType)
throws LambdaConversionException {
AbstractValidatingLambdaMetafactory mf;
mf = new InnerClassLambdaMetafactory(caller, invokedType,
invokedName, samMethodType,
implMethod, instantiatedMethodType,
false, EMPTY_CLASS_ARRAY, EMPTY_MT_ARRAY);
mf.validateMetafactoryArgs();
//此处会创建一个内部类
return mf.buildCallSite();
}- MethodHandles.Lookup caller : 代表查找上下文与调用者的访问权限, 使用invokedynamic指令时, JVM会自动自动填充这个参数
- String invokedName : 要实现的方法的名字, 使用invokedynamic时, JVM自动帮我们填充(填充内容来自常量池InvokeDynamic.NameAndType.Name), 在这里JVM为我们填充为 "apply", 即Consumer.accept方法名.
- MethodType invokedType : 调用点期望的方法参数的类型和返回值的类型(方法signature). 使用invokedynamic指令时, JVM会自动自动填充这个参数(填充内容来自常量池InvokeDynamic.NameAndType.Type), 在这里参数为String, 返回值类型为Consumer, 表示这个调用点的目标方法的参数为String, 然后invokedynamic执行完后会返回一个即Consumer实例.
- MethodType samMethodType : 函数对象将要实现的接口方法类型, 这里运行时, 值为 (Object)Object 即 Consumer.accept方法的类型(泛型信息被擦除).#67 (Ljava/lang/Object;)V
- MethodHandle implMethod : 一个直接方法句柄(DirectMethodHandle), 描述在调用时将被执行的具体实现方法 (包含适当的参数适配, 返回类型适配, 和在调用参数前附加上捕获的参数), 在这里为 #70 invokestatic Lambda.lambda$0:(Ljava/lang/String;)V 方法的方法句柄.
- MethodType instantiatedMethodType : 函数接口方法替换泛型为具体类型后的方法类型, 通常和 samMethodType 一样, 不同的情况为泛型:
4 5 6 三个参数
#27 (Ljava/lang/Object;)Ljava/lang/Object;
#28 invokestatic java/lang/Integer.parseInt:(Ljava/lang/String;)I
#29 (Ljava/lang/String;)Ljava/lang/Integer;//此处会创建一个内部类
return mf.buildCallSite();生成的内部类如下
加上这句话
System.setProperty("jdk.internal.lambda.dumpProxyClasses", "E:\\WorkSpace\\subject\\lambda\\");会在指定目录生成内部类,如下
final class StreamTest$$Lambda$1 implements Function {
private StreamTest$$Lambda$1() {
}
@Hidden
public Object apply(Object var1) {
return Integer.parseInt((String)var1);
}
}如果是这样的代码
new Thread(() -> System.out.println("线程:" + Thread.currentThread().getId())).start();生成的内部类长这样
final class StreamTest$$Lambda$1 implements Runnable {
private StreamTest$$Lambda$1() {
}
@Hidden
public void run() {
StreamTest.lambda$main$0();
}
}这样呢
Consumer consumer = Integer::parseInt; 就长这样
final class StreamTest$$Lambda$1 implements Consumer {
private StreamTest$$Lambda$1() {
}
@Hidden
public void accept(Object var1) {
Integer.parseInt((String)var1);
}
}这样呢
public static void main(String[] args) {
Func add = (x, y) -> x + y;
System.out.println(add.exec(1, 2));
}
@FunctionalInterface
interface Func {
int exec(int x, int y);
}就长这样
final class StreamTest$$Lambda$1 implements Func {
private StreamTest$$Lambda$1() {
}
@Hidden
public int exec(int var1, int var2) {
return StreamTest.lambda$main$0(var1, var2);
}
}3.2 30在这
代表执行哪个方法
#30 = NameAndType #40:#41 // apply:()Ljava/util/function/Function;总结
Lambda表达式，使用invokedynamic指令，运行时调用LambdaMetafactory.metafactory动态的生成内部类，实现了接口
内部类里的调用方法块并不是动态生成的，只是在原class里已经编译生成了一个静态的方法，内部类只需要调用该静态方法