[GitHub] [skywalking] IanCao edited a comment on issue #2503: avoid reordering

[GitBox <gi...@apache.org>]

IanCao edited a comment on issue #2503: avoid reordering
URL: https://github.com/apache/skywalking/pull/2503#issuecomment-485316051
 
 
   ```
    public static FileWriter get() {
           if (INSTANCE == null) {
               synchronized (CREATE_LOCK) {
                   if (INSTANCE == null) {
                       INSTANCE = new FileWriter();
                   }
               }
           }
           return INSTANCE;
       }
   ```
   
   this is the FileWriter double check instantiation code.
   
   `uniqueInstance = new Singleton()` actually process in three steps
   1. allocate memory for `uniqueInstance ` reference
   2. initialize `uniqueInstance ` 
   3. make step1 point step 2
   
   but jvm has reorder characteristics， that order maybe change to 1，3，2
   for example, 
   Thread 1 process step 1 and 3
   at the same time,  Thread 2 process `getUniqueInstance()`  and result is not null, but this time `uniqueInstance` stiil not be initialized.

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