GuardedBy
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.

Example:

import com.google.errorprone.annotations.concurrent.GuardedBy;

class Account {
  @GuardedBy("this")
  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'.
  }

  @GuardedBy("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.

Reference

Lock expression syntax

The following syntax can be used to describe a lock:

Annotations

@GuardedBy

com.google.errorprone.annotations.concurrent.GuardedBy

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();

@GuardedBy("lock")
int x;

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

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 com.google.errorprone.annotations.concurrent.GuardedBy.

@LockMethod

com.google.errorprone.bugpatterns.threadsafety.annotations.LockMethod

The method to which this annotation is applied acquires one or more locks. The caller will hold the locks when the function finishes execution.

This annotation does not apply to implicit locks, which cannot be acquired without being released in the same method.

final Lock lock = new ReentrantLock();

@LockMethod("lock")
acquireLock() {}  // error: 'lock' was not acquired

@UnlockMethod

com.google.errorprone.bugpatterns.threadsafety.annotations.UnlockMethod

The method to which this annotation is applied releases one or more locks. The caller must hold the locks when the function is entered, and will not hold them when it completes.

This annotation does not apply to implicit locks, which cannot be released without being acquired in the same method.

final Lock lock = new ReentrantLock();

@GuardedBy("lock")
int x;

@UnlockMethod("lock")
void releaseLock() {
  lock.unlock();
}

@LockMethod("lock")
void acquireLock() {
  lock.lock();
}

public void increment() {
  acquireLock();
  try {
    x++;  // OK: 'lock' is held
  } finally {
    releaseLock();
  }
}

Limitations

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

class Transaction {
  @GuardedBy("this")
  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.

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) {
  r.run();
}

… 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
  someExecutor.execute(r);
}

False negatives with aliasing

class Names {
  @GuardedBy("this")
  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.

Suppression

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