When @Overrides of synchronized methods are not themselves synchronized, the result can be improper synchronization as callers rely on the thread-safety promised by the parent class.

Noncompliant Code Example

public class Parent {

  synchronized void foo() {
    //...
  }
}

public class Child extends Parent {

 @Override
  public foo () {  // Noncompliant
    // ...
    super.foo();
  }
}

Compliant Solution

public class Parent {

  synchronized void foo() {
    //...
  }
}

public class Child extends Parent {

  @Override
  synchronized foo () {
    // ...
    super.foo();
  }
}

See