Single case mode (double check lock)

Use DCL Dual retrieval mode ：
The code is as follows ：

package SingLeton_pattern;
//import static com.sun.org.apache.xalan.internal.xsltc.compiler.util.Type.String;
// Double check lock / Double check lock （DCL, namely  double-checked locking）

public class Singleton {

    private volatile static Singleton singleton;

    private Singleton(){}

    public static Singleton getSingleton()
    {
        if(singleton == null)
        {
            synchronized (Singleton.class)
            {
                if(singleton == null)
                    singleton = new Singleton();
            }
        }
        return singleton;
    }

}

The following code is not executed atomically in a multithreaded environment ：

singleton = new Singleton();

The normal low-level execution sequence will change into three steps ：

(1)  to Singleton Class singleton Allocate memory 

(2)  Call the instance singleton To initialize member variables 

(3)  take singleton Point to the allocated memory address 

The three steps above , No matter in A To which step is the thread currently executing , Yes B Threads may see A There can only be two states 1,2 All I see is null,3 See non null, That's fine .
But if there is an instruction rearrangement , namely （3） First executed. , that B The thread is in the first null When checking , It is possible to see that this example is not null, At this time, there will be problems if you use it directly .

So here we use ：

volatile Instruction reordering is disabled , So the initialization sequence must be 1->2->3, Therefore, there is no case of getting uninitialized object references .

It was used volatile Modifier member variable , Then after the variable assignment , There will be a memory barrier . That is to say, only after the implementation 1,2,3 After step operation , Read operation can see , Read operations are not reordered before write operations .

stay 《 In depth understanding of Java virtual machine 》 It is described in a book ：

“ Observe the addition of volatile Keywords and did not add volatile Keyword generated assembly code found , Join in volatile When a keyword , One more lock Prefix instruction ”

lock The prefix instruction is actually a memory barrier （ It's also a memory fence ）, The memory barrier will provide 3 Features ：
advantage ：

Keep it up DCL, Relatively simple
determine ：volatile This keyword will bring some performance impact . 

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