LambdaMetafactory的Java访问bean方法 [英] Java access bean methods with LambdaMetafactory

问题描述

我的问题与该主题中的明确使用LambdaMetafactory
密切相关，提供了一些非常好的例子来使用LambdaMetafactory来访问类的静态方法;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。似乎很难找到一个例子，我执行的每一次尝试都以非工作代码结束。

my question is strongly related to Explicit use of LambdaMetafactory in that thread, some very good examples are provided to use the LambdaMetafactory to access a static method of a class; however, I wonder what is the equivalent code to access a non static field of an existing bean instance. It seems really hard to find an example and every attempt I performed ended up in non working code.

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的工作单元测试getStaticObj（）：

Here a working unit test that successfully access the static method "getStaticObj()":

    @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（）方法失败了：

Now here my failing attempts to access the non static "getObj()" method:

    @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负面结果，我真的希望有人是帮助我让至少一个测试工作正常。

Every attempt has a different negative result, I really hope someone is abe to help me to have at least one test working fine.

推荐答案

如果你想将值绑定到你的lamba，你必须将这些参数包含在invokedtype签名中：

If you want to bind values to your lamba, you have to include these parameters to the invokedtype signature:

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<而不是绑定值code>函数以bean作为参数：

---

Instead of binding a value you may use a Function which takes the bean as argument:

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.generic(), target, func);

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

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