[英]Java.lang.reflect.Proxy returning another proxy from invocation results in ClassCastException on assignment
So I'm playing with geotools and I thought I'd proxy one of their data-access classes and trace how it was being used in their code. 所以我正在玩geotools,我想我会代理他们的一个数据访问类,并跟踪它们在代码中的使用方式。
I coded up a dynamic proxy and wrapped a FeatureSource (interface) in it and off it went happily. 我编写了一个动态代理并在其中包含了一个FeatureSource(接口),并且很高兴。 Then I wanted to look at some of the transitive objects returned by the featureSource as well, since the main thing a FeatureSource does is return a FeatureCollection (FeatureSource is analogous to a sql DataSource and featurecollection to an sql statement). 然后我想看一下featureSource返回的一些传递对象,因为FeatureSource做的主要事情是返回一个FeatureCollection(FeatureSource类似于sql DataSource和featurecollection到sql语句)。
in my invocationhandler I just passed the call through to the underlying object, printing out the target class/method/args and result as I went, but for calls that returned a FeatureCollection (another interface), I wrapped that object in my proxy (the same class but a new instance, shouldn't matter should it?) and returned it. 在我的invocationhandler中,我只是将调用传递给底层对象,打印出目标类/方法/ args并按原样结果,但对于返回FeatureCollection(另一个接口)的调用,我将该对象包装在我的代理中(相同的类,但一个新的实例,应该不重要吗?)并返回它。 BAM! BAM! Classcast exception: Classcast异常:
java.lang.ClassCastException: $Proxy5 cannot be cast to org.geotools.feature.FeatureCollection
at $Proxy4.getFeatures(Unknown Source)
at MyClass.myTestMethod(MyClass.java:295)
the calling code: 调用代码:
FeatureSource<SimpleFeatureType, SimpleFeature> featureSource = ... // create the FS
featureSource = (FeatureSource<SimpleFeatureType, SimpleFeature>) FeatureSourceProxy.newInstance(featureSource, features);
featureSource.getBounds();// ok
featureSource.getSupportedHints();// ok
DefaultQuery query1 = new DefaultQuery(DefaultQuery.ALL);
FeatureCollection<SimpleFeatureType, SimpleFeature> results = featureSource.getFeatures(query1); //<- explosion here
the Proxy: 代理人:
public class FeatureSourceProxy implements java.lang.reflect.InvocationHandler {
private Object target;
private List<SimpleFeature> features;
public static Object newInstance(Object obj, List<SimpleFeature> features) {
return java.lang.reflect.Proxy.newProxyInstance(
obj.getClass().getClassLoader(),
obj.getClass().getInterfaces(),
new FeatureSourceProxy(obj, features)
);
}
private FeatureSourceProxy(Object obj, List<SimpleFeature> features) {
this.target = obj;
this.features = features;
}
public Object invoke(Object proxy, Method m, Object[] args)throws Throwable{
Object result = null;
try {
if("getFeatures".equals(m.getName())){
result = interceptGetFeatures(m, args);
}
else{
result = m.invoke(target, args);
}
}
catch (Exception e) {
throw new RuntimeException("unexpected invocation exception: " + e.getMessage(), e);
}
return result;
}
private Object interceptGetFeatures(Method m, Object[] args) throws Exception{
return newInstance(m.invoke(target, args), features);
}
} }
Is it possible to dynamically return proxies of interfaces from a proxied interface or am I doing something wrong? 是否有可能从代理接口动态返回接口代理,或者我做错了什么? cheers! 干杯!
Class.getInterfaces() returns only the interfaces DIRECTLY implemented by the class. Class.getInterfaces()仅返回类实现的DIRECTLY接口。 You need a transitive closure to optain all the interfaces. 您需要一个传递闭包来获取所有接口。
UPDATE UPDATE
Example: 例:
private static Class<?>[] getInterfaces(Class<?> c) {
List<Class<?>> result = new ArrayList<Class<?>>();
if (c.isInterface()) {
result.add(c);
} else {
do {
addInterfaces(c, result);
c = c.getSuperclass();
} while (c != null);
}
for (int i = 0; i < result.size(); ++i) {
addInterfaces(result.get(i), result);
}
return result.toArray(new Class<?>[result.size()]);
}
private static void addInterfaces(Class<?> c, List<Class<?>> list) {
for (Class<?> intf: c.getInterfaces()) {
if (!list.contains(intf)) {
list.add(intf);
}
}
}
You may also need to "unwrapp" the proxies that are passed as arguments. 您可能还需要“解开”作为参数传递的代理。
@maurice-perry's solution worked great for me and I have voted for it, but I did also want to point out that there are library implementations of the needed method. @ maurice-perry的解决方案对我很有用,我已经投了赞成票,但我也想指出有所需方法的库实现。
I ended up implementing this solution with the Apache Commons library method ClassUtils.getAllInterfaces()
: 我最终使用Apache Commons库方法ClassUtils.getAllInterfaces()
实现了这个解决方案:
...
import org.apache.commons.lang3.ClassUtils;
...
private static Class<?>[] getAllInterfaces(Object object) {
final List<Class<?>> interfaces =
ClassUtils.getAllInterfaces(object.getClass());
return interfaces.toArray(new Class<?>[interfaces.size()]);
}
It works great for that magical second argument in newProxyInstance
: 它适用于newProxyInstance
中神奇的第二个参数:
Proxy.newProxyInstance(ClassLoader loader, Class<?>[] interfaces,
InvocationHandler h)
There is also a Guava approach using: 还有一种番石榴方法使用:
final Set<TypeToken> tt = TypeToken.of(cls).getTypes().interfaces();
But then you have to figure out howto convert Set<TypeToken>
to Class<?>[]
. 但是你必须弄清楚如何将Set<TypeToken>
转换为Class<?>[]
。 Trivial perhaps, if you're a Guava buff, but Apache's is ready for use. 也许是微不足道的,如果你是番石榴buff,但是Apache已经可以使用了。
Both of these were noted in this related thread, get all (derived) interfaces of a class . 这两个都在这个相关的线程中被注意到, 获得了类的所有(派生)接口 。
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