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);
Lambda知识整理
Lambda定义
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);
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；
结尾
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