In-depth singleton mode (understanding understanding)

1 /***2 * 1. The simplest implementation3  * 4  * 5  */6 7 /*8 Public class Singletonclass {9 Private Singletonclass () {}Ten private static final Singletonclass instance=new Singletonclass (); One private static Singletonclass getinstance () { A return instance; -     } -  the } - */ - /** -  *  + * 2. Performance optimization -  * +  */ A /*Public class singletonclass{ at Private Singletonclass () {} - private static Singletonclass instance=null; - Public static Singletonclass getinstance () { - if (instance==null) { - instance=new Singletonclass (); -         } in return instance; -     } to }*/ + /*** -  *  the * 3, synchronous, multi-threading *  * $  */Panax Notoginseng /*Public class singletonclass{ - Private Singletonclass () { the     } + private static Singletonclass instance=null; A Public synchronized Static Singletonclass getinstance () { the if (instance==null) { + instance=new Singletonclass (); -         } $ return instance; $          -     } - } the     */ - /***Wuyi * 4. Also performance the  */ - /*Public class singletonclass{ Wu Private Singletonclass () {} - private static Singletonclass instance=null; About Public static Singletonclass getinstance () { $ synchronized (singletonclass.class) { - if (instance==null) { - instance=new Singletonclass (); -             } A         } + return instance; the     } - } $ */ the //double-checked Locking Design the /*Public class singletonclass{ the Private Singletonclass () {} the private static Singletonclass instance=null; - Public Singletonclass getinstance () { in if (instance==null) { the synchronized (singletonclass.class) { the if (instance==null) { About instance=new Singletonclass (); the                 } the             }             the         } + return instance; -     } the }*/Bayi /*** the * 5. Check from source the  */ - /** -  *  the * What steps are required to create a variable? One is to apply a piece of memory, call the constructor method to initialize the operation, and the other is to allocate the * One pointer points to this memory. Which of the two operations who are in front of who in the post? The JVM specification is not specified. So there is a situation, the * The JVM first creates a piece of memory, then points the pointer to the memory, and finally calls the constructor method to initialize it. Let's consider the following. the * In such a situation: Thread A begins to create an instance of Singletonclass, at which point thread B calls the getinstance () method, first - * Determine if instance is null. According to the memory model we mentioned above, A has pointed instance to the memory, just the * The constructor method has not been called, so B detects that instance is not NULL and then returns the instance directly--the problem arises, the * Although instance is not NULL, it is not constructed, like a house has given you the key, but you can't live in it, the * Because the inside hasn't been cleaned up yet. At this point, if B is using this instance before a instance construct is completed, the program will get an error! 94  * the  */ the /*Public class Singletonclass{//wrong job the Private Singletonclass () {}98 private static Singletonclass instance=null; About Public static Singletonclass getinstance () { - if (instance==null) {101 Singletonclass SC;102 synchronized (singletonclass.class) {103 sc=instance;104 if (sc==null) { the synchronized (singletonclass.class) {106 if (sc==null) {107 sc=new Singletonclass ();108                         }109                     } the                 }111 INSTANCE=SC;  the             }113              the         } the return instance; the     }    117 }*/118 119 /*** -  * 121 * 6. Solution122  *123  */124 //after JDK 5, Java uses a new memory model. The volatile keyword has a definite semantics--before JDK1.5, the //volatile is a keyword, but it does not explicitly specify its purpose-the volatile-modified write variable cannot be adjusted to the previous read-write code .126 //read variable can not and after the reading and writing code adjustment! So, as long as we simply add the volatile keyword to the instance. 127 /*Public class singletonclass{ - Private Singletonclass () {}129 private volatile static singletonclass instance=null; the Public static Singletonclass getinstance () {131 if (instance==null) { the synchronized (singletonclass.class) {133 if (instance==null) {134 instance=new Singletonclass ();135                 }            136             }        137         }138 return instance;139     } $ }*/141 /***142 * Recommended Usage143  */144 /*Public class singletonclass{145 Private Singletonclass () {}146 static inner class of//java147 private static Class singletonclassinstance{148 private static final Singletonclass instance=new Singletonclass ();149     } Max Public static Singletonclass getinstance () {151 return singletonclassinstance.instance; the     }    153 }154 */

In-depth singleton mode (understanding understanding)