一、Lambda演进
小王在公司正在开发一个学生管理系统，产品经理向他提出一个需求，要筛选出年龄大于15的学生,于是小王写出了以下代码:
public static List<Student> filterAgeStudent(List<Student> students) { List<Student> list = Lists.newArrayList(); for (Student student : students) { if (student.getAge() > 15) { list.add(student); } } return list; }
过了几天产品经理又提出了一个需求，要筛选出体重大于50KG的学生，于是小王新增了一个方法:
public static List<Student> filterWeightStudent(List<Student> students) { List<Student> list = Lists.newArrayList(); for (Student student : students) { if (student.getWeight() > 50) { list.add(student); } } return list; }
过了一段时间，产品提出了要筛选出体重大于50并且年龄要大于15的学生，小王突然感觉到这不是一个简单的需求，于是小王仔细思考了一下，突然想到将每种筛选的策略抽象成为一个接口，并且将这个接口当做一个参数传入方法中，这样每次就可以只新增策略，其他代码不需要更改了，这样就满足了软件设计的六大原则的开放闭合原则，于是乎诞生以下的设计和代码:
public interface StudentPredicate { boolean filter(Student student); } public class AgeStudentPredicate implements StudentPredicate { @Override public boolean filter(Student student) { return student.getAge() > 20 ? true : false; } } public static List<Student> filterStudent(List<Student> students, StudentPredicate predicate) { List<Student> list = Lists.newArrayList(); for (Student student : students) { if (predicate.filter(student)) { list.add(student); } } return list; }
经过一段时间的学习，小王接触到匿名类，于是小王代码进行更改，以后再也不需要写策略了:
List<Student> list = filterStudent(students, new StudentPredicate() { @Override public boolean filter(Student student) { return student.getAge() > 15; } });
学习到匿名类以后，小王感觉到Java的浩瀚，然后继续学习，后来接触到Lambda，于是对待做了以下改造:
List<Student> list = filterStudent(students, student -> student.getAge() > 15);
二、Lambda知识整理
从上面的演进过程，我们基本上可以得到Lambda表达式是一种匿名函数，简单地说，它是没有声明的方法，也即没有访问修饰符、返回值声明和名字。Java中的Lambda表达式通常使用(argument) -> (body)语法书写，常用的Lamda表达式例子有:
(int a, int b) -> { return a + b; } () -> System.out.println("Hello World"); (String s) -> { System.out.println(s); }
函数式接口
函数式接口指的是是只包含一个抽象方法声明的接口。例如java.lang.Runnable就是一种函数式接口，在 Runnable接口中只声明了一个抽象方法方法void run();
@FunctionalInterface public interface Runnable { public abstract void run(); }
对于注解@FunctionalInterface对于lamda表达式来说的话并不是必要的，@FunctionalInterface是Java8新加入的一种接口，用于指明该接口类型声明是根据Java语言规范定义的函数式接口。Java8还声明了一些Lambda表达式可以使用的函数式接口，当你注释的接口不是有效的函数式接口时，可以使用@FunctionalInterface解决编译层面的错误。
常用函数式
Java8中在java.util.function中引入了很多的函数式接口，这里介绍一下3个常用的函数式接口，
- Predicate
Predicate接口定义一个名叫test的抽象方法，它接收泛型T对象，并返回一个boolean类型。经常使用到的地方是在流处理的过程中filter方法，满足条件的数据才会被过滤出来,例如我们上面的例子也可以改造成为Predicate函数式接口的形式。
@FunctionalInterface public interface Predicate<T> { boolean test(T t); } public static List<Student> filterStudent(List<Student> students, Predicate<Student> predicate) { List<Student> list = Lists.newArrayList(); for (Student student : students) { if (predicate.test(student)) { list.add(student); } } return list; }
- Consumer
Consumer定义一个名叫accept的抽象方法，他接受泛型T的对象，没有返回值。如果你需要访问泛型对象T，并其进行修改，就使用Consumer。经常使用的地方就是常用的forEach方法。
@FunctionalInterface public interface Consumer<T> { void accept(T t); } void forEachOrdered(Consumer<? super T> action);
- Function
Function定义一个叫apply的方法，他接受一个泛型对象T，返回一个泛型对象R。如果你需要定义一个Lambda表达式，将输入的对象映射到输出，就使用Function，经常使用到的地方就是常用的map方法。
@FunctionalInterface public interface Function<T, R> { R apply(T t); } <R> Stream<R> map(Function<? super T, ? extends R> mapper);
三、Lambda原理窥探
小王经过上面一系列学习，开始思考Lambda的原理是什么，因为Java8中每一个Lambda表达式必须有一个函数式接口与之对应，小王就思考经过编译器编译以后到可能实现的方式有两种，一种生成实现接口的类，另外一种是内部类，于是决定看一下反编译的以后代码，以解除心中的疑惑；
@FunctionalInterface public interface Func { int add(int x, int y); } public class LambdaTest { public static void main(String[] args) { Func func = (x, y) -> x + y; System.out.println(func.add(1, 2)); } }
通过javap -p -v -c LambdaTest.class查看反编译后的代码，
Classfile /Users/wangtongzhou/Documents/Java/learning/target/classes/com/springboot2/learning/javabasic/java8/LambdaTest.class Last modified 2020-7-11; size 1392 bytes MD5 checksum ec7d77a8b0b0a0cb5940f80a9b27b3d0 Compiled from "LambdaTest.java" public class com.springboot2.learning.javabasic.java8.LambdaTest minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #7.#29 // java/lang/Object."<init>":()V #2 = InvokeDynamic #0:#34 // #0:add:()Lcom/springboot2/learning/javabasic/java8/Func; #3 = Fieldref #35.#36 // java/lang/System.out:Ljava/io/PrintStream; #4 = InterfaceMethodref #37.#38 // com/springboot2/learning/javabasic/java8/Func.add:(II)I #5 = Methodref #39.#40 // java/io/PrintStream.println:(I)V #6 = Class #41 // com/springboot2/learning/javabasic/java8/LambdaTest #7 = Class #42 // java/lang/Object #8 = Utf8 <init> #9 = Utf8 ()V #10 = Utf8 Code #11 = Utf8 LineNumberTable #12 = Utf8 LocalVariableTable #13 = Utf8 this #14 = Utf8 Lcom/springboot2/learning/javabasic/java8/LambdaTest; #15 = Utf8 main #16 = Utf8 ([Ljava/lang/String;)V #17 = Utf8 args #18 = Utf8 [Ljava/lang/String; #19 = Utf8 func #20 = Utf8 Lcom/springboot2/learning/javabasic/java8/Func; #21 = Utf8 MethodParameters #22 = Utf8 lambda$main$0 #23 = Utf8 (II)I #24 = Utf8 x #25 = Utf8 I #26 = Utf8 y #27 = Utf8 SourceFile #28 = Utf8 LambdaTest.java #29 = NameAndType #8:#9 // "<init>":()V #30 = Utf8 BootstrapMethods #31 = MethodHandle #6:#43 // invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; #32 = MethodType #23 // (II)I #33 = MethodHandle #6:#44 // invokestatic com/springboot2/learning/javabasic/java8/LambdaTest.lambda$main$0:(II)I #34 = NameAndType #45:#46 // add:()Lcom/springboot2/learning/javabasic/java8/Func; #35 = Class #47 // java/lang/System #36 = NameAndType #48:#49 // out:Ljava/io/PrintStream; #37 = Class #50 // com/springboot2/learning/javabasic/java8/Func #38 = NameAndType #45:#23 // add:(II)I #39 = Class #51 // java/io/PrintStream #40 = NameAndType #52:#53 // println:(I)V #41 = Utf8 com/springboot2/learning/javabasic/java8/LambdaTest #42 = Utf8 java/lang/Object #43 = Methodref #54.#55 // java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; #44 = Methodref #6.#56 // com/springboot2/learning/javabasic/java8/LambdaTest.lambda$main$0:(II)I #45 = Utf8 add #46 = Utf8 ()Lcom/springboot2/learning/javabasic/java8/Func; #47 = Utf8 java/lang/System #48 = Utf8 out #49 = Utf8 Ljava/io/PrintStream; #50 = Utf8 com/springboot2/learning/javabasic/java8/Func #51 = Utf8 java/io/PrintStream #52 = Utf8 println #53 = Utf8 (I)V #54 = Class #57 // java/lang/invoke/LambdaMetafactory #55 = NameAndType #58:#62 // metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; #56 = NameAndType #22:#23 // lambda$main$0:(II)I #57 = Utf8 java/lang/invoke/LambdaMetafactory #58 = Utf8 metafactory #59 = Class #64 // java/lang/invoke/MethodHandles$Lookup #60 = Utf8 Lookup #61 = Utf8 InnerClasses #62 = Utf8 (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; #63 = Class #65 // java/lang/invoke/MethodHandles #64 = Utf8 java/lang/invoke/MethodHandles$Lookup #65 = Utf8 java/lang/invoke/MethodHandles { public com.springboot2.learning.javabasic.java8.LambdaTest(); descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: return LineNumberTable: line 3: 0 LocalVariableTable: Start Length Slot Name Signature 0 5 0 this Lcom/springboot2/learning/javabasic/java8/LambdaTest; public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=4, locals=2, args_size=1 0: invokedynamic #2, 0 // InvokeDynamic #0:add:()Lcom/springboot2/learning/javabasic/java8/Func; 5: astore_1 6: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream; 9: aload_1 10: iconst_1 11: iconst_2 12: invokeinterface #4, 3 // InterfaceMethod com/springboot2/learning/javabasic/java8/Func.add:(II)I 17: invokevirtual #5 // Method java/io/PrintStream.println:(I)V 20: return LineNumberTable: line 5: 0 line 6: 6 line 7: 20 LocalVariableTable: Start Length Slot Name Signature 0 21 0 args [Ljava/lang/String; 6 15 1 func Lcom/springboot2/learning/javabasic/java8/Func; MethodParameters: Name Flags args private static int lambda$main$0(int, int); descriptor: (II)I flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC Code: stack=2, locals=2, args_size=2 0: iload_0 1: iload_1 2: iadd 3: ireturn LineNumberTable: line 5: 0 LocalVariableTable: Start Length Slot Name Signature 0 4 0 x I 0 4 1 y I MethodParameters: Name Flags x synthetic y synthetic } SourceFile: "LambdaTest.java" InnerClasses: public static final #60= #59 of #63; //Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles BootstrapMethods: 0: #31 invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; Method arguments: #32 (II)I #33 invokestatic com/springboot2/learning/javabasic/java8/LambdaTest.lambda$main$0:(II)I
反编译以后lambda表达式被编译成为一个lambda$main$0的函数，其实就是一段(x, y) -> x + y的方法，在看main方法主要分为以下8个步骤:
- 通过invokedynamic指令生成调用对象；
- 存入本地缓存；
- 加载java.lang.System.out静态方法；
- 将lambda表达式生成的对象加载入执行栈;
- 将int类型1加载入执行栈;
- 将int类型2加载入执行栈;
- 执行lambda表达式生成的对象的add方法；
- 输出执行结果；
重点部分
从mian方法中我们的重点就在于invokedynamic这个指令，重点要了解下是如何通过invokedynamic指令生成目标对象，invokedynamic指令通过找到BootstrapMethods中的方法，生成动态调用点，也是调用LambdaMetafactory的metafactory方法。
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(); return mf.buildCallSite(); }
通过源码可以看出，metafactory方法通过InnerClassLambdaMetafactory类生成对象，并提供后续调用，在InnerClassLambdaMetafactory源码中可以看到，有提供开关是否dump生成的class文件。
接下来我们通过设置启动参数-Djdk.internal.lambda.dumpProxyClasses查看中间对象，增加这个参数以后会生成LambdaTest$$Lambda$1类,
final class LambdaTest$$Lambda$1 implements Func { private LambdaTest$$Lambda$1() { } @Hidden public int add(int var1, int var2) { return LambdaTest.lambda$main$0(var1, var2); } }
我们再看下上面这个类反编译以后的情况
Classfile /Users/wangtongzhou/Documents/Java/learning/com/springboot2/learning/javabasic/java8/LambdaTest$$Lambda$1.class Last modified 2020-7-11; size 437 bytes MD5 checksum 729979930540708c60f4e71e63b69321 final class com.springboot2.learning.javabasic.java8.LambdaTest$$Lambda$1 implements com.springboot2.learning.javabasic.java8.Func minor version: 0 major version: 52 flags: ACC_FINAL, ACC_SUPER, ACC_SYNTHETIC Constant pool: #1 = Utf8 com/springboot2/learning/javabasic/java8/LambdaTest$$Lambda$1 #2 = Class #1 // com/springboot2/learning/javabasic/java8/LambdaTest$$Lambda$1 #3 = Utf8 java/lang/Object #4 = Class #3 // java/lang/Object #5 = Utf8 com/springboot2/learning/javabasic/java8/Func #6 = Class #5 // com/springboot2/learning/javabasic/java8/Func #7 = Utf8 <init> #8 = Utf8 ()V #9 = NameAndType #7:#8 // "<init>":()V #10 = Methodref #4.#9 // java/lang/Object."<init>":()V #11 = Utf8 add #12 = Utf8 (II)I #13 = Utf8 Ljava/lang/invoke/LambdaForm$Hidden; #14 = Utf8 com/springboot2/learning/javabasic/java8/LambdaTest #15 = Class #14 // com/springboot2/learning/javabasic/java8/LambdaTest #16 = Utf8 lambda$main$0 #17 = NameAndType #16:#12 // lambda$main$0:(II)I #18 = Methodref #15.#17 // com/springboot2/learning/javabasic/java8/LambdaTest.lambda$main$0:(II)I #19 = Utf8 Code #20 = Utf8 RuntimeVisibleAnnotations { private com.springboot2.learning.javabasic.java8.LambdaTest$$Lambda$1(); descriptor: ()V flags: ACC_PRIVATE Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial #10 // Method java/lang/Object."<init>":()V 4: return public int add(int, int); descriptor: (II)I flags: ACC_PUBLIC Code: stack=2, locals=3, args_size=3 0: iload_1 1: iload_2 2: invokestatic #18 // Method com/springboot2/learning/javabasic/java8/LambdaTest.lambda$main$0:(II)I 5: ireturn RuntimeVisibleAnnotations: 0: #13() }
由此我们可以得出编译以后的代码为：
public class LambdaTest { public static void main(String[] args) { Func func= LambdaTest$$Lambda$1(); System.out.println(func.add(1, 2)); } private static int lambda$main$0(int x, int y) { return x + y; } static final class LambdaTest$$Lambda$1 implements Func { private LambdaTest$$Lambda$1() { } public int add(int x, inty) { return LambdaTest.lambda$main$0(x,y); } } }
总结下，Lambda底层就是通过一个静态的内部类实现的；
