/**
 * 单例模式（线程不安全）
 */
public class SingletonTest {

    private static SingletonTest instance;

    private SingletonTest() {
    }

    public static SingletonTest getInstance() {
        if (instance == null) {
            instance = new SingletonTest();
        }
        return instance;
    }
}

/**
 * 单例模式（线程安全、每次调用都同步getInstance方法，影响效率）
 */
public class SingletonTest {

    private static SingletonTest instance;

    private SingletonTest() {
    }

    public static synchronized SingletonTest getInstance() {
        if (instance == null) {
            instance = new SingletonTest();
        }
        return instance;
    }
}

/**
 * 单例模式（线程安全、JVM加载类时马上创建实例）
 */
public class SingletonTest {

    private static SingletonTest instance = new SingletonTest();

    private SingletonTest() {
    }

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

/**
 * 单例模式（线程安全）
 */
public class SingletonTest {

    private volatile static SingletonTest instance;

    private SingletonTest() {
    }

    public static SingletonTest getInstance() {
        if (instance == null) {
            synchronized (SingletonTest.class) {
                if(instance == null) {
                    instance = new SingletonTest();
                }
            }
        }
        return instance;
    }
}