我正在尝试显式使用 LambdaMetafactory.metafactory，我不明白为什么它只适用于 Runnable 功能接口(interface)。例如，此代码执行预期的操作(打印“hello world”):

public class MetafactoryTest {
    
    public static void main(String[] args) throws Throwable {

        MethodHandles.Lookup caller = MethodHandles.lookup();
        MethodType methodType = MethodType.methodType(void.class);
        MethodType invokedType = MethodType.methodType(Runnable.class);
        CallSite site = LambdaMetafactory.metafactory(caller, 
                                                      "run", 
                                                      invokedType, 
                                                      methodType, 
                                                      caller.findStatic(MetafactoryTest.class, "print", methodType), 
                                                      methodType);
        MethodHandle factory = site.getTarget();
        Runnable r = (Runnable) factory.invoke();
        r.run();
    }
    
    private static void print() {
        System.out.println("hello world"); 
    }    
}

当我尝试使用不同的功能接口(interface)(例如 Supplier)时，问题就出现了。以下代码不起作用:

public class MetafactoryTest {

    public static void main(String[] args) throws Throwable {

        MethodHandles.Lookup caller = MethodHandles.lookup();
        MethodType methodType = MethodType.methodType(String.class);
        MethodType invokedType = MethodType.methodType(Supplier.class);
        CallSite site = LambdaMetafactory.metafactory(caller, 
                                                      "get", 
                                                      invokedType, 
                                                      methodType, 
                                                      caller.findStatic(MetafactoryTest.class, "print", methodType), 
                                                      methodType);
        MethodHandle factory = site.getTarget();
        Supplier<String> r = (Supplier<String>) factory.invoke();
        System.out.println(r.get());        
    }
    private static String print() {
        return "hello world";
    }    
}


Exception in thread "main" java.lang.AbstractMethodError: metafactorytest.MetafactoryTest$$Lambda$1/258952499.get()Ljava/lang/Object;
    at metafactorytest.MetafactoryTest.main(MetafactoryTest.java:29)

这两段代码不应该以类似的方式工作吗，第二段的问题是什么？

此外，下面的代码应该是等价的，效果很好:

public class MetafactoryTest {

    public static void main(String[] args) throws Throwable {
        Supplier<String> r = (Supplier<String>) () -> print();
        System.out.println(r.get());        
    }

    private static String print() {
        return "hello world";
    }    
}

编辑

另一种避免改变方法返回类型的解决方案是定义一个新的功能接口(interface):

public class MetafactoryTest {

    @FunctionalInterface
    public interface Test {
        String getString();
    }
    
    public static void main(String[] args) throws Throwable {

        MethodHandles.Lookup caller = MethodHandles.lookup();
        MethodType methodType = MethodType.methodType(String.class);
        MethodType invokedType = MethodType.methodType(Test.class);
        CallSite site = LambdaMetafactory.metafactory(caller, 
                                                      "getString", 
                                                      invokedType, 
                                                      methodType, 
                                                      caller.findStatic(MetafactoryTest.class, "print", methodType), 
                                                      methodType);
        MethodHandle factory = site.getTarget();
        Test r = (Test) factory.invoke();
        System.out.println(r.getString());        
    }
    
    private static String print() {
        return "hello world";
    }  

最佳答案

Runnable 和 Supplier 的区别在于 Supplier 使用的是泛型。

在运行时，Supplier 没有 String get() 方法，它有 Object get()。但是你实现的方法返回一个字符串。您需要区分这两种类型。像这样:

public class MetafactoryTest {

    public static void main(String[] args) throws Throwable {

        MethodHandles.Lookup caller = MethodHandles.lookup();
        MethodType methodType = MethodType.methodType(Object.class);
        MethodType actualMethodType = MethodType.methodType(String.class);
        MethodType invokedType = MethodType.methodType(Supplier.class);
        CallSite site = LambdaMetafactory.metafactory(caller, 
                                                      "get", 
                                                      invokedType, 
                                                      methodType, 
                                                      caller.findStatic(MetafactoryTest.class, "print", actualMethodType), 
                                                      methodType);
        MethodHandle factory = site.getTarget();
        Supplier<String> r = (Supplier<String>) factory.invoke();
        System.out.println(r.get());
    }

    private static String print() {
        return "hello world";
    }    
}

关于java - 显式使用 LambdaMetafactory，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/26775676/