明确使用LambdaMetafactory

Question

我试图显式地使用LambdaMetafactory.metafactory，我不明白为什么它只能与Runnable功能接口一起使用。例如，以下代码完成了预期的工作（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"); 
    }    
}

当我尝试使用其他功能接口（例如供应商）时，就会出现问题。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)

这两个代码段不应该以相似的方式工作吗，这是第二个代码段中的问题？

此外，以下应等效的代码也可以正常工作：

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

避免更改方法返回类型的另一种解决方案是定义一个新的功能接口：

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";
    }

Answer 1

Runnable和Supplier之间的区别在于Supplier使用通用类型。

在运行时，供应商没有String get（）方法，而有Object get（）。但是您实现的方法返回一个String。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";
    }    
}