Java 摘要中的延迟加载单例 Class

Question

我正在尝试在 Java 中使用抽象 class 实现单例实例。我读过像将它实现为 LazyLoad 是最佳实践。我做不到，因为我不太习惯这种模式，甚至 Java.

getInstance() 始终为 null，我不确定为什么？
我不认为这是在构造实例时遵循延迟加载。
除此之外，欢迎任何其他评论，因为我是 Java 的新手并且之前使用过 c#。

这是我的，

接口：

public interface IConditionAppender{
    public String Append();
}

摘要

public abstract AppenderBase {

    private static IConditionAppender instance;

    protected AppenderBase(IConditionAppender instance)
    {
         this.instance = instance;
    }

    public static IConditionAppender getInstance(){ return instance; }
}

实施

public final class AndAppender extends AppenderBase implements IConditionAppender {

private AndAppender()
{
    super(new AndAppender())
}

@Override
public String Append()
{
    return " AND ";
}

}

测试

System.out.println(AndAppender.getInstance().Append());

Answer 1

下面是 AndAppender 的实现方式

public final class AndAppender implements ConditionAppender {
    private static final AndAppender instance;
    public static AndAppender getInstance() {
        if (instance == null)
            instance = new AndAppender();
        return instance;
    }

    private AndAppender() { }

    @Override
    public String append() {
        return " AND ";
    }
}

OrAppender 等

的方法相同

注意：此实现不是线程安全的。

更简单一点的方法是使用Enum，默认是单例，可以实现接口

public enum  Appender implements ConditionAppender {
    AND(" AND "), OR(" OR ");

    final String operation;

    Appender(String operation) {
        this.operation = operation;
    }

    @Override
    public String append() {
        return operation;
    }

    public static void main(String[] args) {
        System.out.println(AND.append());
        System.out.println(OR.append());

    }
}

Answer 2

下面的代码或许能帮到你~

public abstract class AbstractSingleton {

    private static Map<String, AbstractSingleton> registryMap = new HashMap<String, AbstractSingleton>();

    AbstractSingleton() throws SingletonException {
        String clazzName = this.getClass().getName();
        if (registryMap.containsKey(clazzName)) {
            throw new SingletonException("Cannot construct instance for class " + clazzName + ", since an instance already exists!");
        } else {
            synchronized (registryMap) {
                if (registryMap.containsKey(clazzName)) {
                    throw new SingletonException("Cannot construct instance for class " + clazzName + ", since an instance already exists!");
                } else {
                    registryMap.put(clazzName, this);
                }
            }
        }
    }

    @SuppressWarnings("unchecked")
    public static <T extends AbstractSingleton> T getInstance(final Class<T> clazz) throws InstantiationException, IllegalAccessException {
        String clazzName = clazz.getName();
        if (!registryMap.containsKey(clazzName)) {
            synchronized (registryMap) {
                if (!registryMap.containsKey(clazzName)) {
                    T instance = clazz.newInstance();
                    return instance;
                }
            }
        }
        return (T) registryMap.get(clazzName);
    }

    public static AbstractSingleton getInstance(final String clazzName)
            throws ClassNotFoundException, InstantiationException, IllegalAccessException {
        if (!registryMap.containsKey(clazzName)) {
            Class<? extends AbstractSingleton> clazz = Class.forName(clazzName).asSubclass(AbstractSingleton.class);
            synchronized (registryMap) {
                if (!registryMap.containsKey(clazzName)) {
                    AbstractSingleton instance = clazz.newInstance();
                    return instance;
                }
            }
        }
        return registryMap.get(clazzName);
    }

    @SuppressWarnings("unchecked")
    public static <T extends AbstractSingleton> T getInstance(final Class<T> clazz, Class<?>[] parameterTypes, Object[] initargs)
            throws SecurityException, NoSuchMethodException, IllegalArgumentException,
            InvocationTargetException, InstantiationException, IllegalAccessException {
        String clazzName = clazz.getName();
        if (!registryMap.containsKey(clazzName)) {
            synchronized (registryMap) {
                if (!registryMap.containsKey(clazzName)) {
                    Constructor<T> constructor = clazz.getConstructor(parameterTypes);
                    T instance = constructor.newInstance(initargs);
                    return instance;
                }
            }
        }
        return (T) registryMap.get(clazzName);
    }

    static class SingletonException extends Exception {
        private static final long serialVersionUID = -8633183690442262445L;

        private SingletonException(String message) {
            super(message);
        }
    }
}

发件人：https://www.cnblogs.com/wang9192/p/3975748.html

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