Checks for unguarded accesses to fields and methods with @GuardedBy annotations


Alternate names: GuardedByChecker

The problem

The GuardedBy analysis checks that fields or methods annotated with @GuardedBy(lock) are only accessed when the specified lock is held.



class Account {
  private int balance;

  public synchronized int getBalance() {
    return balance; // OK: implicit 'this' lock is held.

  public synchronized void withdraw(int amount) {
    setBalance(balance - amount); // OK: implicit 'this' lock is held.

  public void deposit(int amount) {
    setBalance(balance + amount); // ERROR: access to 'balance' not guarded by 'this'.

  private void setBalance(int newBalance) {
    checkState(newBalance >= 0, "Balance cannot be negative.");
    balance = newBalance; // OK: 'this' must be held by caller of 'setBalance'.

This above example uses implicit locks (via the ‘synchronized’ modifier). The analysis also supports synchronized statements and java.util.concurrent locks.

Basic Concepts

The analysis provides a way of associating members with locks. A member is a field or a method. A lock can be the implicit lock of an object, or a java.util.concurrent Lock.

An implicit lock is acquired using the built in synchronization features of the language. Adding the ‘synchronized’ modifier to an instance method causes the implicit lock of the enclosing instance to be acquired for the duration of the method. Adding the ‘synchronized’ modifier to a static method is similar, except the implicit lock of the Class object is acquired instead.

The Locks defined in java.util.concurrent are acquired with explicit lock()/unlock() methods. The use of these methods in Java should always correspond to a try/finally block, to ensure that the locks are released on all execution paths.


Lock expression syntax

The following syntax can be used to describe a lock:

this The implicit object lock of the enclosing class.
ClassName.this The implicit object lock of the enclosing class specified by ClassName. (For inner classes, the ClassName.this designation allows you to specify which 'this' reference is intended.)
The final instance field specified by fieldName.
The instance method specified by methodName(). Methods called to return locks should be deterministic.
ClassName.class The implicit lock of specified Class object.
ClassName.fieldName The static final field specified by fieldName.
ClassName.methodName() The static method specified by methodName(). Methods called to return locks should be deterministic.
itself The annotated field.



The @GuardedBy annotation is used to document that a member (a field or a method) can only be accessed when the specified lock is held.

@GuardedBy can be used with both implicit locks and java.util.concurrent Locks.

final Lock lock = new ReentrantLock();

int x;

void m() {
  x++;  // error: access of 'x' not guarded by 'lock'
  try {
    x++;  // OK: guarded by 'lock'
  } finally {

Note: there are a couple more annotations called @GuardedBy, including javax.annotation.concurrent.GuardedBy and org.checkerframework.checker.lock.qual.GuardedBy. The check recognizes those versions of the annotation, but we recommend using


Anonymous classes and lambdas need to re-acquire locks that may be held by an enclosing block. For example, consider:

class Transaction {
  int x;

  public synchronized void handle() {
    doSomething(() -> {
      x++;  // Error: access of 'x' not guarded by 'Transaction.this'

The analysis is intra-procedural, meaning it doesn’t consider the implementation of doSomething.

In general, the checker doesn’t know if doSomething immediately calls the provided lambda while the lock is still held by the enclosing method handle, for example:

private void doSomething(Runnable r) {;

… or whether the lambda could be called later after handle has released the lock, for example:

private void doSomething(Runnable r) {
  // runs `r` at some point in the future

However, the check does special-case some method calls which are known to immediately call the provided lambda or method reference.

False negatives with aliasing

class Names {
  List<String> names = new ArrayList<>();

  public void addName(String name) {
    List<String> copyOfNames;
    synchronized (this) {
      copyOfNames = names;  // OK: access of 'names' guarded by 'this'
    copyOfNames.add(name);  // should be an error: this access is not thread-safe!

The analysis does not track aliasing, so it’s possible to circumvent the safety it provides by copying references to guarded members.

In the example, the guarded field ‘names’ can be accessed via a copy even if the required lock is not held.


Suppress false positives by adding the suppression annotation @SuppressWarnings("GuardedBy") to the enclosing element.