设计模式:惰性单例、泛型和继承:java.lang.Class不能强制转换为java.lang.reflect.Para

Question

我正在尝试实现泛型和继承的懒惰单例。我已经创建了一个abstract超类，并声明了一个Map，它将存储该类子类的所有实例。

在这里:

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;
        }
    }
}

其中一个子类是:

public class ActivityTypeXMLParser extends AbstractXMLParser<ActivityTypeXMLParser>{
    private ActivityTypesXMLModel activityTypes;
    private ActivityTypeXMLParser() {
    }
    public static void main(String... strings) throws InstantiationException, IllegalAccessException {
        ActivityTypeXMLParser.getInstance();
    }
}

main法用于测试。但是我得到例外:

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)

现在调试后，我发现

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

是异常的原因，因为AbstractXMLParser.class.getClass().getGenericSuperclass()返回class java.lang.Object。我需要在这里得到Class<T>。我如何在AbstractXMLParser类和AbstractXMLParser的SingletonHolder中提取它?

Answer 1

这段代码有多个问题:

• static方法不被继承。当ActivityTypeXMLParser.getInstance()被调用时，AbstractXMLParser.getInstance()不知道它是在ActivityTypeXMLParser的上下文中被调用的

• AbstractXMLParser.class.getClass()返回你的类的Class' of类, not类

• 不清楚为什么你需要AbstractXMLParser<T>是通用的。注意AbstractXMLParser、SingletonHolder、getInstance()中的T s是不同的类型参数

您需要隐式地将有问题的类传递给getInstance():

getInstance(ActivityTypeXMLParser.class);

.

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

Answer 2

这是我做的完整的解决方案，可能对别人有帮助。

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);
        }
    }
}

和其中一个子类:

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

您似乎想要获得类的泛型类型，该类是抽象类的子类。然而，你得到

AbstractXMLParser.class.getClass()

这是抽象类的类，它总是Class，而你应该只使用getClass()来表示具体类的类。