In Java, it's possible Singleton per thread?

Question

Using threads, I have a principal class (SlaveCrawler) that instantiates three classes(Downloader, ContentAnalyzer, URLAnalyzer) , which are dependent on each other.

SlaveCrawler uses Downloader and URLAnalyzer

Downloader uses ContentAnalyzer and URLAnalyzer

ContentAnalyzer uses URLAnalyzer

I want only one instance of each class. If I use Singleton, I can get this, but working with threads, I will have 20 SlaveCrawlers(example), so I want 20 URLAnalyzer.

It's possible make this using Singleton or I need other way?

Answer 1

Take a look at ThreadLocal. Each thread will have its own local copy of each object.

ThreadLocal<YourObject> threadLocalYourObject = new ThreadLocal<YourObject>() {
  @Override
  protected YourObject initialValue() {
    //initialize YourObject
  }
}

Or in 1.8 we can use:

ThreadLocal<YourObject> threadLocalYourObject = ThreadLocal.withInitial( () -> new YourObject() )

To get access to your ThreadLocal object, use the get() method.

YourObject yourObject = threadLocalYourObject.get();

Answer 2

You can implement it with ThreadLocal. Here is the pseudo code:

public class ThreadLocalTest {

  public static void main(String[] args){
    MyTLSingleTon obj = MyTLSingleTon.getInstance();
  }

}

class MyTLSingleTon{

  private MyTLSingleTon(){  
  }

  private static final ThreadLocal<MyTLSingleTon> _localStorage = new ThreadLocal<MyTLSingleTon>(){
    protected MyTLSingleTon initialValue() {
      return new MyTLSingleTon();
   }
  };

  public static MyTLSingleTon getInstance(){
    return _localStorage.get();
  }
}

MyTLSingleTon.getInstance(); method will return an object assosiated with the current thread. and if no object is assosiated than protected MyTLSingleTon initialValue() method will be called and a new instance will be set.