Write to static field from instance method

问题:

I have my code as below. I see

public MyClass{      private static DataSource dataSource = null;      private static DataSource getDataSource(){         if (dataSource == null) {             try {                 dataSource = // something.             } catch (Exception e) {                 // some exception.             }         }          return dataSource;     }      public List doSomething(){          // ...          if(dataSource == null){             dataSource = getDataSource();         }          dataSource.getConnection();         // ...      } } 

I see following message in sonar anaylsis.

Dodgy - Write to static field from instance method  This instance method writes to a static field. This is tricky to get correct if multiple instances are being manipulated, and generally bad practice. findbugs:ST_WRITE_TO_STATIC_FROM_INSTANCE_METHOD Sep12 Reliability > Architecture 

I see everything is okay in this implementation except that we are changing the static variable in doSomething method. How do we fix this ?

回答1:

Not sure how your static analysis tool works but -

try writing to your value via a static setter:

private synchronized static void setDataSource(DataSource ds) {     dataSource = ds; } 

so that you can do

   if(dataSource == null){         setDataSource(getDataSource());    } 

回答2:

You have to choose between two solutions , first one is in your Datasource class add getDatasource() method and synchronize the block where you instantiate your static field :

    private static DataSource getDataSource(){      synchronize(DataSource.class){             if (dataSource == null) {                 try {                     dataSource = // something.                 } catch (Exception e) {                     // some exception.                 }             }        }         return dataSource;       } 

then you will just call this method from your doSomething() method

public void doSomething(){ // DataSource dataSource=DataSource.getDataSource(); // } 

second solution is to instantiate your field at the class loading time using a static block or directly from the declaration.

static {  dataSource = // something. } 

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