设计模式：Lazy Singleton，Generics和Inheritance：java.lang.Class无法强制转换为java.lang.reflect.ParameterizedType

Question

I am trying to implement Lazy Singleton with Generics and Inheritance. I have created a abstract super-class and declared a Map which will store all the instances of the child class of this class.

Here it is:

public abstract class AbstractXMLParser<T> {
    @SuppressWarnings("rawtypes")
    private static final Map<Class<? extends AbstractXMLParser>, AbstractXMLParser> INSTANCES = new HashMap<>();

    public AbstractXMLParser() {
        throw new UnsupportedOperationException("Cannot instantiate");
    }

    private static class SingletonHolder<T> {       
        @SuppressWarnings({ "unchecked"})
        private static <T> T getInstance() throws InstantiationException, IllegalAccessException {
            Class<T> clazz = (Class<T>) ((ParameterizedType) SingletonHolder.class.getClass().getGenericSuperclass()).getActualTypeArguments()[0];
            return clazz.newInstance();
        }
    }

    @SuppressWarnings({ "unchecked", "rawtypes" })
    public static <T extends AbstractXMLParser> T getInstance() throws InstantiationException, IllegalAccessException {
        Class<T> clazz = (Class<T>) ((ParameterizedType) AbstractXMLParser.class.getClass().getGenericSuperclass()).getActualTypeArguments()[0];
        if(INSTANCES.containsKey(clazz)) {
            return (T) INSTANCES.get(clazz);
        } else {
            T instance = SingletonHolder.getInstance();
            INSTANCES.put(clazz, instance);
            return instance;
        }
    }
}

And one of the child class is:

public class ActivityTypeXMLParser extends AbstractXMLParser<ActivityTypeXMLParser>{
    private ActivityTypesXMLModel activityTypes;

    private ActivityTypeXMLParser() {

    }

    public static void main(String... strings) throws InstantiationException, IllegalAccessException {
        ActivityTypeXMLParser.getInstance();

    }
}

The main method is for testing purpose. But I am getting exception:

Exception in thread "main" java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType
    at com.edfx.adb.xml.parser.AbstractXMLParser.getInstance(AbstractXMLParser.java:25)
    at com.edfx.adb.xml.parser.ActivityTypeXMLParser.main(ActivityTypeXMLParser.java:30)

Now after debugging I found that

Class<T> clazz = (Class<T>) ((ParameterizedType) AbstractXMLParser.class.getClass().getGenericSuperclass()).getActualTypeArguments()[0];

is the reason for the exception, because AbstractXMLParser.class.getClass().getGenericSuperclass() is returning class java.lang.Object . I need to get the Class<T> here. How can I extract it in the class AbstractXMLParser and in SingletonHolder of AbstractXMLParser ?

Answer 1

There are multiple issues with this code:

• static methods are not inherited. AbstractXMLParser.getInstance() doesn't know that it's invoked in context of ActivityTypeXMLParser when called as ActivityTypeXMLParser.getInstance() .

• AbstractXMLParser.class.getClass() returns Class' of Class , not Class` of your class.

• It's not clear why do you need AbstractXMLParser<T> to be generic at all. Note that T s in AbstractXMLParser , SingletonHolder and getInstance() are different type parameters.

You need to pass the class in question to getInstance() implicitly:

getInstance(ActivityTypeXMLParser.class);

.

@SuppressWarnings({ "unchecked", "rawtypes" })
public static <T extends AbstractXMLParser> T getInstance(Class<T> clazz) 
    throws InstantiationException, 
    ...
}

Answer 2

Here is the complete solution that I have made, would be helpful for someone.

The abstract super-class :

@SuppressWarnings({"all"})
public abstract class AbstractXMLParser<T> {    
    private static final Map<Class<? extends AbstractXMLParser>, AbstractXMLParser> INSTANCES = new HashMap<>();

    public AbstractXMLParser() {

    }

    private static class SingletonHolder<T> {               
        private static <T> T getInstance(Class<T> clazz) throws InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {
            Constructor<T> constructor = (Constructor<T>) clazz.getDeclaredConstructors()[0];
            constructor.setAccessible(true);            
            return constructor.newInstance(null);
        }
    }

    protected static <T extends AbstractXMLParser<T>> T getInstance(Class<T> clazz) throws InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {      
        if(INSTANCES.containsKey(clazz)) {
            return (T) INSTANCES.get(clazz);
        } else {
            T instance = SingletonHolder.getInstance(clazz);
            INSTANCES.put(clazz, instance);
            return instance;
        }
    }

    protected static <T extends AbstractXMLParser<T>> void putInstance(Class<T> clazz, T instance) {
        if(!INSTANCES.containsKey(clazz)) {
            INSTANCES.put(clazz, instance);
        }
    }
}

And one of the child class:

public class ActivityTypeXMLParser extends AbstractXMLParser<ActivityTypeXMLParser>{

    private ActivityTypeXMLParser() {

    }   

    public static ActivityTypeXMLParser getInstance() {
        ActivityTypeXMLParser activityTypeXMLParser = null;

        try {
            activityTypeXMLParser = getInstance(ActivityTypeXMLParser.class);
        } catch (Exception exception) {
        }

        if(activityTypeXMLParser == null) {
            activityTypeXMLParser = new ActivityTypeXMLParser();
            putInstance(ActivityTypeXMLParser.class, activityTypeXMLParser);
        }

        return activityTypeXMLParser;
    }
}

Answer 3

You appear to want to get the generic type of the class which is a sub class of the abstract class. However you are getting

AbstractXMLParser.class.getClass()

This is class of the abstract class which is always Class Instead you should use just getClass() to the the class of the concrete class.