带有 LambdaMetafactory 的 Java 访问 bean 方法

Question

我的问题与Explicit use of LambdaMetafactory 密切相关 在该线程中，提供了一些非常好的示例来使用 LambdaMetafactory 访问类的静态方法；但是，我想知道访问现有 bean 实例的非静态字段的等效代码是什么。似乎真的很难找到一个例子，而且我执行的每一次尝试都以非工作代码告终。

这是 bean 代码：

class SimpleBean {
    private Object obj= "myCustomObject";
    private static Object STATIC_OBJECT = "myCustomStaticObject";
    public Object getObj() {
        return obj;
    }
    public void setObj(final Object obj) {
        this.obj = obj;
    }
    public static Object getStaticObj() {
        return STATIC_OBJECT;
    }
    public static void setStaticObj(final Object obj) {
        STATIC_OBJECT = obj;
    }
}

这是一个成功访问静态方法“getStaticObj()”的工作单元测试：

    @Test
public void accessStaticMethod() throws Throwable
{
    MethodHandles.Lookup caller = MethodHandles.lookup();
    Method reflected = SimpleBean.class.getDeclaredMethod("getStaticObj");
    MethodHandle methodHandle = caller.unreflect(reflected);
    CallSite site = LambdaMetafactory.metafactory(caller,
            "get",
            MethodType.methodType(Supplier.class),
            MethodType.methodType(Object.class),
            methodHandle,
            MethodType.methodType(Object.class));
    MethodHandle factory = site.getTarget();
    Supplier r = (Supplier) factory.invoke();
    assertEquals( "myCustomStaticObject", r.get());
}

现在我尝试访问非静态“getObj()”方法失败：

    @Test
public void accessNonStaticMethodTestOne() throws Throwable
{
    SimpleBean simpleBeanInstance = new SimpleBean();

    MethodHandles.Lookup caller = MethodHandles.lookup();
    MethodHandle methodHandle = caller.bind(simpleBeanInstance, "getObj", MethodType.methodType(Object.class));
    assertEquals("myCustomObject", methodHandle.invoke());

    // This test fails here with exception:
    // java.lang.IllegalArgumentException: not a direct method handle
    CallSite site = LambdaMetafactory.metafactory(caller,
            "get",
            MethodType.methodType(Supplier.class),
            MethodType.methodType(Object.class),
            methodHandle,
            MethodType.methodType(Object.class));

    MethodHandle factory = site.getTarget();
    Supplier r = (Supplier) factory.invoke();
    assertEquals( "myCustomObject", r.get());

}

@Test
public void accessNonStaticMethodTwo() throws Throwable
{

    SimpleBean simpleBeanInstance = new SimpleBean();

    MethodHandles.Lookup caller = MethodHandles.lookup();

    Method reflected = SimpleBean.class.getDeclaredMethod("getObj");
    MethodHandle methodHandle = caller.unreflect(reflected);

    // This test fails here with exception:
    // java.lang.invoke.LambdaConversionException: Incorrect number of parameters
    CallSite site = LambdaMetafactory.metafactory(caller,
            "get",
            MethodType.methodType(Supplier.class),
            MethodType.methodType(Object.class),
            methodHandle,
            MethodType.methodType(Object.class));

    MethodHandle factory = site.getTarget();
    factory = factory.bindTo(simpleBeanInstance);
    Supplier r = (Supplier) factory.invoke();
    assertEquals( "myCustomObject", r.get());

}


@Test
public void accessNonStaticMethodThree() throws Throwable
{

    SimpleBean simpleBeanInstance = new SimpleBean();

    MethodHandles.Lookup caller = MethodHandles.lookup();

    Method reflected = SimpleBean.class.getDeclaredMethod("getObj");
    MethodHandle methodHandle = caller.unreflect(reflected);

    CallSite site = LambdaMetafactory.metafactory(caller,
            "get",
            MethodType.methodType(Supplier.class),
            MethodType.methodType(Object.class, SimpleBean.class),
            methodHandle,
            MethodType.methodType(Object.class, SimpleBean.class));

    MethodHandle factory = site.getTarget();

    //This test fails here with exception:
    // java.lang.IllegalArgumentException: no leading reference parameter: spike.LambdaBeanAccessAtRuntimeTest$SimpleBean@4459eb14
    factory = factory.bindTo(simpleBeanInstance);
    Supplier r = (Supplier) factory.invoke();
    assertEquals( "myCustomObject", r.get());

}

每次尝试都有不同的阴性结果，我真的希望有人能帮助我让至少一个测试正常工作。

Answer 1

如果您想将值绑定到您的 Lamba，您必须将这些参数包含到调用类型签名中：

SimpleBean simpleBeanInstance = new SimpleBean();

MethodHandles.Lookup caller = MethodHandles.lookup();
MethodType getter=MethodType.methodType(Object.class);
MethodHandle target=caller.findVirtual(SimpleBean.class, "getObj", getter);
CallSite site = LambdaMetafactory.metafactory(caller,
    "get", // include types of the values to bind:
    MethodType.methodType(Supplier.class, SimpleBean.class),
    getter, target, getter);

MethodHandle factory = site.getTarget();
factory = factory.bindTo(simpleBeanInstance);
Supplier r = (Supplier) factory.invoke();
assertEquals( "myCustomObject", r.get());

---

您可以使用 Function 来代替绑定值，它将 bean 作为参数：

SimpleBean simpleBeanInstance = new SimpleBean();

MethodHandles.Lookup caller = MethodHandles.lookup();
MethodType getter=MethodType.methodType(Object.class);
MethodHandle target=caller.findVirtual(SimpleBean.class, "getObj", getter);
MethodType func=target.type();
CallSite site = LambdaMetafactory.metafactory(caller,
    "apply",
    MethodType.methodType(Function.class),
    func.erase(), target, func);

MethodHandle factory = site.getTarget();
Function r = (Function)factory.invoke();
assertEquals( "myCustomObject", r.apply(simpleBeanInstance));