单例模式使得一个类的实例是唯一的,外部对它的访问都针对同一个对象。 单例模式的使用可以是业务上的原因,比如一个User对象需要全局唯一,或者是性能上的考虑,避免创建代价较高的对象反复实例化。
public class User {
private static final User ourInstance = new User();
public static User getInstance() {
return ourInstance;
}
private User() {
}
}
public class User {
private static volatile User ourInstance;
public static User getInstance() {
if (ourInstance == null) {
synchronized (User.class) {
if (ourInstance == null) {
ourInstance = new User();
}
}
}
return ourInstance;
}
private User() {
}
}
public class User {
public static User getInstance() {
return Holder.ourInstance;
}
private static class Holder {
private static User ourInstance = new User();
}
private User() {
}
}
public abstract class Singleton<T> {
private T mInstance;
protected abstract T create();
public final T get() {
synchronized (this) {
if (mInstance == null) {
mInstance = create();
}
return mInstance;
}
}
}
使用:
private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
protected IActivityManager create() {
IBinder b = ServiceManager.getService("activity");
if (false) {
Log.v("ActivityManager", "default service binder = " + b);
}
IActivityManager am = asInterface(b);
if (false) {
Log.v("ActivityManager", "default service = " + am);
}
return am;
}
};
好处就是对无法修改的目标类型进行单例化,好像也没多大卵用。
public enum User{
INSTANCE;
}
public Object readResolve(){
return getInstance();
}