Java动态适配器

Question

我正在使用一些生成以下接口(interface)的 Java 代码。此代码不可修改。

interface A1 {
    public void run();
}

interface A2 {
    public void run();
}

...

interface A24 {
    public void run();
}

我在使用以下代码时遇到类转换错误。我如何动态地构建一个适配器到我的界面？

interface ARunnable {
    public void run();
}

public void someMethod() {
    // getARunnables() returns a list of A1, A2, ... A24
    List<ARunnable> runnables = (List<ARunnable>)getARunnables(); 

    for (ARunnable a : runnables) {
        a.run();
    }
}

Answer 1

由于无法修改接口(interface)来扩展 java.lang.Runnable，因此可以选择使用 java.lang.reflect.Proxy 来构建委托(delegate)给 A1、A2... 接口(interface)的 Runnable 实例。

这并不简单，但请看一下使用 java.lang.reflect.Proxy 的示例。该示例只是根据方法名称委托(delegate)给委托(delegate)对象。

public class ProxyTest
{
    public static void main(String... args)
    {
        List<?> instances = Arrays.asList(new A1());
        List<ARunnable> runnableInstances = new ArrayList<ARunnable>(instances.size());
        for (Object instance : instances)
        {
            ARunnable runnableInstance = (ARunnable)Proxy.newProxyInstance(
                                            ARunnable.class.getClassLoader(), 
                                            new Class<?>[] {ARunnable.class}, 
                                            new RunnableWrapper(instance));
            runnableInstances.add(runnableInstance);
        }

        //Now we have a list of ARunnables!
        //Use them for something
        for (ARunnable runnableInstance : runnableInstances)
        {
            runnableInstance.run();
        }
    }

    private static class RunnableWrapper implements InvocationHandler
    {
        private final Object instance;

        public RunnableWrapper(Object instance)
        {
            this.instance = instance;
        }

        @Override
        public Object invoke(Object proxy, Method method, Object[] args)
                throws Throwable
        {
            //Ensure that your methods match exactly or you'll get NoSuchMethodExceptions here
            Method delegateMethod = instance.getClass().getMethod(method.getName(), method.getParameterTypes());
            return(delegateMethod.invoke(instance, args));
        }
    }

    public static class A1
    {
        public void run()
        {
            System.out.println("Something");
        }
    }

    public static interface ARunnable
    {
        public void run();
    }
}

我还建议你修复线路

List<ARunnable> runnables = (List<ARunnable>)getARunnables(); 

您不应忽视这种类型的安全警告。该对象列表实际上并不包含 ARunnables，因此会出现 ClassCastException。