synchronized and monitors

Intermediate Algorithms and Data Structures English

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Created by Best · 16.08.2026 at 09:13 UTC

synchronized in Java locks a monitor (object). Only one thread holds the lock. Visibility and atomic sections are thus ensured (happens-before).

Coarse locks can slow down performance; fine locks are prone to errors.

Where used

Classic JVM mutual exclusion. This is foundational before moving on to java.util.concurrent, locks, and lock-free structures.

Depth

Any Java object can serve as a monitor. A synchronized block acquires its intrinsic lock, executes the protected section, and releases the lock upon exit, even in the case of exceptions. Only code that uses the same monitor collectively protects the same invariant.

Exiting a monitor publishes previous changes; later acquiring the same monitor makes them visible. Synchronized thus regulates both mutual exclusion and memory order. Choosing a private, stable lock object prevents unintended external lock coupling.

Difficulty levels

Determine the monitor of an instance method and a static method.
Protect all accesses to an invariant with the same lock.
Balance the scope of locking between safety and parallelism.

Pitfalls

Two different lock objects do not protect against each other. Publicly accessible or changing lock objects are also risky. Multiple locks in inconsistent order can lead to a deadlock.

University approvals: 0

Tasks

Question 1

What does synchronized do on an object?

GC is starting.

It serializes the entry into the protected section.

It clears the heap.

It sorts arrays.

Question 2

Two methods protect the same field with two different lock objects. What is the problem?

The locks do not mutually exclude their critical sections.

Java automatically unites all lock objects.

The field becomes immutable as a result.

Both methods always run in the same thread.

Question 3

Implement LockedBox. set and get should use the object's monitor.

Hint

A synchronized block on the current object is written as synchronized (this) { ... }.

class LockedBox {
  private int value;

  // TODO: Zugriff synchronisieren
  void set(int value) {
    this.value = value;
  }

  int get() {
    return value;
  }
}

Starter code is prefilled; replace TODO blocks with your solution.

Runtime output (stdout/stderr)

1 test case will be used for grading

Run checks runtime behavior only. Final correctness is evaluated when you submit.

Card Info

Topic: Algorithms and Data Structures
Difficulty: Intermediate
Completed: 0 users

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