Do not synchronize on the result of `getClass()`JAVA-E1021

Object.getClass() returns the runtime type of a variable.

Because of this, it is not guaranteed that the same object will be returned for all values contained in a variable of a certain type, unless that type is declared as final.

Attempting to synchronize on the result of getClass() called on a variable with a non-final type could lead to concurrency bugs such as race conditions.

Bad Practice

class MyClass {
    // ...
}

class MySubClass extends MyClass {
    // ...
}

// ...

// Both of these values are declared as being of type MyClass...
MyClass firstExample = new MyClass();
MyClass secondExample = new MySubClass();

// firstExample.getClass() returns MyClass.class...
synchronized (firstExample.getClass()) {
    // ...
}

// secondExample.getClass() returns MySubClass.class !!!
synchronized (secondExample.getClass()) {
    // ...
}

While the declared type of secondExample is MyClass, its actual type at runtime is MySubClass. This means that we will not be synchronizing on the same object in the first and second synchronized blocks.

Recommended

Use the static class instance of the specific type you wish to use instead:

synchronized (MyClass.class) {
    // ...
}

Exceptions

This issue will not be raised when the type of the value getClass() is used on is declared as final since a final type cannot be inherited from.

References

• CWE-362 - Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')