由浅入深学习java8的Lambda原理
2017-04-17 本文已影响0人
realqiyan
java8lambda由浅入深，通过一个简单例子，逐步深入了解lambda实现原理。
一个简单的java8中lambda例子
先看一个简单的使用lambda的例子，我们从这个例子开始逐步探索java8中lambda是如何实现的。
/**
* Created by qiyan on 2017/4/16.
*/
public class LambdaTest {
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);
}
上面源码编译完成后执行 javap -p -v -c LambdaTest 查看反编译结果：
Classfile /Users/qiyan/src/test/src/main/java/LambdaTest.class
Last modified 2017-4-16; size 969 bytes
MD5 checksum 0a1db458a90b20fbfae645b576725fd4
Compiled from "LambdaTest.java"
public class LambdaTest
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #7.#18 // java/lang/Object."<init>":()V
#2 = InvokeDynamic #0:#23 // #0:exec:()LFunc;
#3 = Fieldref #24.#25 // java/lang/System.out:Ljava/io/PrintStream;
#4 = InterfaceMethodref #26.#27 // Func.exec:(II)I
#5 = Methodref #28.#29 // java/io/PrintStream.println:(I)V
#6 = Class #30 // LambdaTest
#7 = Class #31 // java/lang/Object
#8 = Utf8 <init>
#9 = Utf8 ()V
#10 = Utf8 Code
#11 = Utf8 LineNumberTable
#12 = Utf8 main
#13 = Utf8 ([Ljava/lang/String;)V
#14 = Utf8 lambda$main$0
#15 = Utf8 (II)I
#16 = Utf8 SourceFile
#17 = Utf8 LambdaTest.java
#18 = NameAndType #8:#9 // "<init>":()V
#19 = Utf8 BootstrapMethods
#20 = MethodHandle #6:#32 // invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#21 = MethodType #15 // (II)I
#22 = MethodHandle #6:#33 // invokestatic LambdaTest.lambda$main$0:(II)I
#23 = NameAndType #34:#35 // exec:()LFunc;
#24 = Class #36 // java/lang/System
#25 = NameAndType #37:#38 // out:Ljava/io/PrintStream;
#26 = Class #39 // Func
#27 = NameAndType #34:#15 // exec:(II)I
#28 = Class #40 // java/io/PrintStream
#29 = NameAndType #41:#42 // println:(I)V
#30 = Utf8 LambdaTest
#31 = Utf8 java/lang/Object
#32 = Methodref #43.#44 // java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#33 = Methodref #6.#45 // LambdaTest.lambda$main$0:(II)I
#34 = Utf8 exec
#35 = Utf8 ()LFunc;
#36 = Utf8 java/lang/System
#37 = Utf8 out
#38 = Utf8 Ljava/io/PrintStream;
#39 = Utf8 Func
#40 = Utf8 java/io/PrintStream
#41 = Utf8 println
#42 = Utf8 (I)V
#43 = Class #46 // java/lang/invoke/LambdaMetafactory
#44 = NameAndType #47:#51 // metafactory:(Ljava/lang/invoke/MethodHandles$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 = NameAndType #14:#15 // lambda$main$0:(II)I
#46 = Utf8 java/lang/invoke/LambdaMetafactory
#47 = Utf8 metafactory
#48 = Class #53 // java/lang/invoke/MethodHandles$Lookup
#49 = Utf8 Lookup
#50 = Utf8 InnerClasses
#51 = Utf8 (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#52 = Class #54 // java/lang/invoke/MethodHandles
#53 = Utf8 java/lang/invoke/MethodHandles$Lookup
#54 = Utf8 java/lang/invoke/MethodHandles
{
public 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 4: 0
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:exec:()LFunc;
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 Func.exec:(II)I
17: invokevirtual #5 // Method java/io/PrintStream.println:(I)V
20: return
LineNumberTable:
line 7: 0
line 8: 6
line 9: 20
private static int lambda$main$0(int, int);
descriptor: (II)I
flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC
Code:
stack=2, locals=2, args_size=2
0: iload_0
1: iload_1
2: iadd
3: ireturn
LineNumberTable:
line 7: 0
}
SourceFile: "LambdaTest.java"
InnerClasses:
public static final #49= #48 of #52; //Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles
BootstrapMethods:
0: #20 invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
Method arguments:
#21 (II)I
#22 invokestatic LambdaTest.lambda$main$0:(II)I
#21 (II)I
根据反编译结果，不难看出lambda表达式：(x, y) -> x + y 被编译成了一个方法：lambda0
private static int lambda$main$0(int, int);
descriptor: (II)I
flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC
Code:
stack=2, locals=2, args_size=2
0: iload_0
1: iload_1
2: iadd
3: ireturn
LineNumberTable:
line 7: 0
翻译成java代码：
private static int lambda$main$0(int x, int y){
return x + y;
}
再看看main方法字节码：
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:exec:()LFunc;
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 Func.exec:(II)I
17: invokevirtual #5 // Method java/io/PrintStream.println:(I)V
20: return
LineNumberTable:
line 7: 0
line 8: 6
line 9: 20
执行main步骤：
0：通过invokedynamic指令生成调用对象；
5：存入本地变量表；
6：加载java.lang.System.out静态方法；
9：将lambda表达式生成的对象加载入执行栈；
10：将int类型1加载入执行栈；
11：将int类型2加载入执行栈；
12：执行lambda表达式生成的对象的exec方法；
17：输出执行结果；
lambda的要点就在invokedynamic这个指令了，通过invokedynamic指令生成目标对象，接下来我们了解一下invokedynamic指令。
invokedynamic指令
下面重点看看invokedynamic指令，首先我们来看看常量池中出现的的InvokeDynamic类型：
官方文档：https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.4.10
CONSTANT_InvokeDynamic_info {
u1 tag;//InvokeDynamic类型标记18
u2 bootstrap_method_attr_index; //BootstrapMethods_attribute中的坐标
u2 name_and_type_index; //名字&类型常量池坐标
}
接下来看看BootstrapMethods_attribute，InvokeDynamic中的bootstrap_method_attr_index就是指向其中bootstrap_methods的下标：
官方文档：https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.21
BootstrapMethods_attribute {
u2 attribute_name_index;
u4 attribute_length;
u2 num_bootstrap_methods;
{ u2 bootstrap_method_ref;//方法引用
u2 num_bootstrap_arguments;//参数数量
u2 bootstrap_arguments[num_bootstrap_arguments];//参数
} bootstrap_methods[num_bootstrap_methods];
}
继续bootstrap_methods中的bootstrap_method_ref：
官方文档：https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.4.8
CONSTANT_MethodHandle_info {
u1 tag;//MethodHandle类型标记15
u1 reference_kind;//方法引用类型getfield／getstatic／putfield／putstatic／invokevirtual／invokestatic／invokespecial／new／invokeinterface，此例中使用的invokestatic。
u2 reference_index;//引用类型，根据reference_kind确认，例如本例中kind为方法调用，所以index为Methodref，细节可以查看官方文档。
}
回归lambda例子
对invokedynamic学习总结一下：
invokedynamic指令通过找到BootstrapMethods中的方法，生成动态调用点，对于本例，我们对照BootstrapMethods中的bootstrap_methods分析实现：
BootstrapMethods:
0: #20 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:
#21 (II)I
#22 invokestatic LambdaTest.lambda$main$0:(II)I
#21 (II)I
按照上面指令，可以看出，invokedynamic指令通过java.lang.invoke.LambdaMetafactory#metafactory方法生成目标对象。
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();
}
查看相关代码可以看出，metafactory就是核心方法，该方法通过InnerClassLambdaMetafactory类生成对象，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。
private static final ProxyClassesDumper dumper;
static {
final String key = "jdk.internal.lambda.dumpProxyClasses";
String path = AccessController.doPrivileged(new GetPropertyAction(key), null,new PropertyPermission(key , "read"));
dumper = (null == path) ? null : ProxyClassesDumper.getInstance(path);
}
//dump逻辑
if (dumper != null) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
@Override
public Void run() {
dumper.dumpClass(lambdaClassName, classBytes);
return null;
}
}, null,
new FilePermission("<<ALL FILES>>", "read, write"),
// createDirectories may need it
new PropertyPermission("user.dir", "read"));
}
执行下面命令生成中间对象java -Djdk.internal.lambda.dumpProxyClasses LambdaTest
Classfile /Users/qiyan/src/test/src/main/java/LambdaTest$$Lambda$1.class
Last modified 2017-4-17; size 236 bytes
MD5 checksum 983fa2b5e7d29c46d6f885925909b83e
final class LambdaTest$$Lambda$1 implements Func
minor version: 0
major version: 52
flags: ACC_FINAL, ACC_SUPER, ACC_SYNTHETIC
Constant pool:
#1 = Utf8 LambdaTest$$Lambda$1
#2 = Class #1 // LambdaTest$$Lambda$1
#3 = Utf8 java/lang/Object
#4 = Class #3 // java/lang/Object
#5 = Utf8 Func
#6 = Class #5 // Func
#7 = Utf8 <init>
#8 = Utf8 ()V
#9 = NameAndType #7:#8 // "<init>":()V
#10 = Methodref #4.#9 // java/lang/Object."<init>":()V
#11 = Utf8 exec
#12 = Utf8 (II)I
#13 = Utf8 LambdaTest
#14 = Class #13 // LambdaTest
#15 = Utf8 lambda$main$0
#16 = NameAndType #15:#12 // lambda$main$0:(II)I
#17 = Methodref #14.#16 // LambdaTest.lambda$main$0:(II)I
#18 = Utf8 Code
{
private LambdaTest$$Lambda$1();
descriptor: ()V
flags: ACC_PRIVATE
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #10 // Method java/lang/Object."<init>":()V
4: return
public int exec(int, int);
descriptor: (II)I
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=3
0: iload_1
1: iload_2
2: invokestatic #17 // Method LambdaTest.lambda$main$0:(II)I
5: ireturn
}
第一个例子学习分析到此结束，下面是根据原理，翻译的等价的最终执行代码。
public class LambdaTest {
public static void main(String[] args) {
Func add = new LambdaTest$$Lambda$1();
System.out.println(add.exec(1, 2));
}
private static int lambda$main$0(int x, int y) {
return x + y;
}
static final class LambdaTest$$Lambda$1 implements Func {
private LambdaTest$$Lambda$1() {
}
public int exec(int x, int y) {
return LambdaTest.lambda$main$0(x, y);
}
}
}
@FunctionalInterface
interface Func {
int exec(int x, int y);
}
后续有时间继续补充变量作用域相关实现原理...
