Java 单例模式的七种写法
第一种（懒汉，线程不安全）
public class Singleton {
private static Singleton instance;
private Singleton() {
}
public static Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
第二种（懒汉，线程安全）
public class Singleton {
private static Singleton instance;
private Singleton() {
}
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
第三种（饿汉）
public class Singleton {
private static final Singleton instance = new Singleton();
private Singleton() {
}
public static Singleton getInstance() {
return instance;
}
}
第四种（饿汉，变种）
public class Singleton {
private static final Singleton instance;
static {
instance = new Singleton();
}
private Singleton() {
}
public static Singleton getInstance() {
return instance;
}
}
第五种（静态内部类）
public class Singleton {
private static final class SingletonHolder {
private static final Singleton INSTANCE = new Singleton();
}
private Singleton() {
}
public static Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}
第六种（枚举）
public enum Singleton {
INSTANCE;
public void whateverMethod() {
}
}
第七种（双重校验锁）
public class Singleton {
private static volatile Singleton singleton;
private Singleton() {
}
public static Singleton getSingleton() {
if (singleton == null) {
synchronized (Singleton.class) {
if (singleton == null) {
singleton = new Singleton();
}
}
}
return singleton;
}
}
两个问题总结
- 对第一个问题修复的办法是
private static Class getClass(String classname) throws ClassNotFoundException {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
if (classLoader == null)
classLoader = Singleton.class.getClassLoader();
return (classLoader.loadClass(classname));
}
- 对第二个问题修复的办法是
public class Singleton implements java.io.Serializable {
public static Singleton INSTANCE = new Singleton();
protected Singleton() {
}
private Object readResolve() {
return INSTANCE;
}
}
参考资料