我把实现concrete class的Null Objects也加上了，是用cglib2来实现的(因此如果你在项目中使用，需要去下载cglib.jar)。看一下源码,注意加粗的部分(注：Pojo中可以不声明constructor，如果有constructors，则必须声明缺省不带任何参数的constructor)：
NUll.java
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import java.lang.reflect.Array;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.HashMap;
import java.util.Map;
import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;
public class NUll implements InvocationHandler, MethodInterceptor {
private Map resultValuesByType = new HashMap();
public NUll() {
createDefaultResults();
}
public static Object object(Class clazz) {
if (clazz.isInterface()) //interface
return Proxy.newProxyInstance(clazz.getClassLoader(),
new Class[] { clazz }, new NUll());
else //concrete class
return Enhancer.create(clazz, new NUll());
}
//implement InvocationHandler
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable {
Class returnType = method.getReturnType();
if (resultValuesByType.containsKey(returnType)) {
return resultValuesByType.get(returnType);
} else if (returnType.isArray()) {
return Array.newInstance(returnType.getComponentType(), 0);
} else if (returnType.isInterface()) {
return NUll.object(returnType);
} else {
return NUll.object(returnType);
}
}
//implement MethodInterceptor
public Object intercept(Object thisProxy, Method method, Object[] args,
MethodProxy superProxy) throws Throwable {
return invoke(thisProxy, method, args);
}
public void addResult(Class resultType, Object resultValue) {
resultValuesByType.put(resultType, resultValue);
}
protected void createDefaultResults() {
addResult(boolean.class, Boolean.FALSE);
addResult(void.class, null);
addResult(byte.class, new Byte((byte) 0));
addResult(short.class, new Short((short) 0));
addResult(int.class, new Integer(0));
addResult(long.class, new Long(0L));
addResult(char.class, new Character('\0'));
addResult(float.class, new Float(0.0F));
addResult(double.class, new Double(0.0));
addResult(Boolean.class, Boolean.FALSE);
addResult(Byte.class, new Byte((byte) 0));
addResult(Short.class, new Short((short) 0));
addResult(Integer.class, new Integer(0));
addResult(Long.class, new Long(0L));
addResult(Character.class, new Character('\0'));
addResult(Float.class, new Float(0.0F));
addResult(Double.class, new Double(0.0));
addResult(String.class, "");
}
}
