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concurrency issues in java

All threads in java have their own call stack but they may share some shared resources. In case of shared resources visibility and access problems can occur.

Visibility problem: Visibility problem occurs when one thread (say first thread) reads a field value which is later updated by some other thread but changes are not guaranteed to be seen by first thread.

class TestThread extends Thread {
  private boolean stop = false;
 
  public void run() {
    while(!stop) {
      testTask();
    }
  }
 
  public void setStop() {
    this.stop = true;
  }
}

Note: If stop is not volatile or setStop and run are not synchronized it is not guaranteed to work.

Access problem: It occurs when multiple threads access and change shared data at same time which may leads in incorrect data.

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