Java singleton 单例

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饿汉式，instance在类加载化时完成初始化，线程安全

package cookie;public class SingletonAtOnce {
private SingletonAtOnce() {
} private static final SingletonAtOnce instance = new SingletonAtOnce(); public static SingletonAtOnce getInstance() {
return instance;
}
}

懒汉式，双锁检查机制保证线程安全

package cookie;public class SingletonDCL {
private SingletonDCL() {
} private static volatile SingletonDCL instance; public static SingletonDCL getInstance() {
if (instance == null) {
synchronized (SingletonDCL.class) {
if (instance == null) {
instance = new SingletonDCL();
}
}
}
return instance;
}
}

私有内部类持有，也属于懒汉式，读ClassHolder中的instance时，导致ClassHolder加载，加载时完成instance的初始化，线程安全

package cookie;public class SingletonClassHolder {
private SingletonClassHolder() {
} private static class Holder {
public static SingletonClassHolder instance = new SingletonClassHolder();
} public static SingletonClassHolder getInstance() {
return Holder.instance;
}
}