关于java的lambda-优快云博客

本文链接：https://blog.youkuaiyun.com/flyforlove/article/details/84896680

本文深入探讨了Java中Lambda表达式的实现原理，通过具体示例解释了Lambda如何被编译器转换为等效的匿名内部类，展示了底层MethodHandle和LambdaMetafactory的使用。

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先看一段代码

public class Lambda {
	public static void main(String[] args) {
                System.setProperty("jdk.internal.lambda.dumpProxyClasses", ".");
		Function<Integer, String> fun1 = String::valueOf;
		Function<String, Integer> fun2 = String::length;
		System.out.println(fun1.apply(1));
		System.out.println(fun2.apply("hello"));
	}

}

那么，javac为我们做了什么呢？等价与那些代码呢？

对于第一种为

	public static void main(String[] args) throws Throwable {
        MethodHandles.Lookup caller = MethodHandles.lookup();
        String invokedName = "apply";
        MethodType invokedType = MethodType.methodType(Function.class);
        MethodHandle implMethod = caller.findStatic(String.class, "valueOf", 
        		MethodType.methodType(String.class,int.class));
        MethodType instantiatedMethodType = MethodType.methodType(String.class,int.class);
        CallSite site = LambdaMetafactory.metafactory(caller,
        					      invokedName, 
                                                      invokedType, 
                                                      instantiatedMethodType.generic(),
                                                      implMethod, 
                                                      instantiatedMethodType);
        MethodHandle factory = site.getTarget();
        Function<Integer,String> fun1 = (Function<Integer,String>) factory.invoke();
        System.out.println(fun1.apply(1));
	}

第二种为

        MethodHandles.Lookup caller = MethodHandles.lookup();
        String invokedName = "apply";
        MethodType invokedType = MethodType.methodType(Function.class);
        MethodHandle implMethod = caller.findVirtual(String.class, "length", 
        		MethodType.methodType(int.class));
        MethodType instantiatedMethodType = MethodType.methodType(int.class,String.class);
        CallSite site = LambdaMetafactory.metafactory(caller,
        					      invokedName, 
                                                      invokedType, 
                                                      instantiatedMethodType.generic(),
                                                      implMethod, 
                                                      instantiatedMethodType);
        MethodHandle factory = site.getTarget();
        Function<String,Integer> fun2 = (Function<String,Integer>) factory.invoke();
        System.out.println(fun2.apply("hello"));

生成的具体类为

第一个

final class Lambda$$Lambda$1
implements Function {
    private Lambda$$Lambda$1() {
    }

    @LambdaForm.Hidden
    public Object apply(Object object) {
        return String.valueOf((Integer)object);
    }
}

第二个

final class Lambda$$Lambda$2
implements Function {
    private Lambda$$Lambda$2() {
    }

    @LambdaForm.Hidden
    public Object apply(Object object) {
        return ((String)object).length();
    }
}