Singleton single-State Design Mode singleton

Singleton single-State Design Mode singleton
1. Concept

1. In Java applications, only one instance of a Class can exist.
2. One instance	1) private constructor 2) Private Static instances 3) Public and static access	1) prevent creating new instances 2) Prevent Modification 3) must be able to obtain

Public class Singleton {// 1. private constructor private Singleton () {}// 2. private static instance variable private static Singleton sl = new Singleton (); // 3. public static access method public static Singleton getsingleton () {return sl ;}}

1. Cool and lazy

1. Evil guys	Create an object from the beginning	1) create an object in advance
2. Lazy	Create an object as needed	1) create an object only when using the method

3. Attackers	4. Lazy
Public class badmash {// 1. private constructor private badmash () {}// 2. private static instance variable private static badmash bmBadmash = new badmash (); // 3. public static access method public static badmash getBadmash () {return bmBadmash ;}}	Public class sluggard {// 1. private constructor private sluggard () {}// 2. private static variable instance private static sluggard sg = null; // 3. public static access method public static sluggard getSluggard () {if (sg = null) {sg = new sluggard () ;}return sg ;}}
1) attackers are thread-safe.	1) The Lazy thread is insecure.

1. Thread Security

1) The role of thread synchronization: control the code and queue the thread for execution,

Synchronization keyword	Synchronous Code Block
Public class sluggard_synchro { // 1. Private Constructor Private sluggard_synchro (){   }   // 2. Private static variable instance Private static sluggard_synchro sg = null;   // 3. Public static access method Public static synchronized sluggard_synchro getSluggard (){ If (sg = null ){ Sg = new sluggard_synchro (); } Return sg; } }	Public class sluggard_code { // 1. Private Constructor Private sluggard_code (){   }   // 2. Private static variable instance Private static sluggard_code sg = null;   // 3. Public static access method Final private static Object OBJECT = new Object ();   Public static sluggard_code getSluggard (){ Synchronized (OBJECT ){ If (sg = null ){ Sg = new sluggard_code (); } } Return sg; } }
Low Efficiency

4. Double Lock

 

Public class sluggard_bilayer { // 1. Private Constructor Private sluggard_bilayer (){   }   // 2. Private static variable instance Private static sluggard_bilayer sg = null;   // 3. Public static access method Final private static Object OBJECT = new Object ();   Public static sluggard_bilayer getSluggard (){ If (sg = null) {// condition 1 Synchronized (OBJECT ){ If (sg = null) {// condition 2 Sg = new sluggard_bilayer (); } } } Return sg; } }

 

Note: Services at the Android system level are generally designed as one instance, that is, a single instance.