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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.checker.lock.qual;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.checkerframework.framework.qual.DefaultFor;
import org.checkerframework.framework.qual.DefaultQualifierInHierarchy;
import org.checkerframework.framework.qual.JavaExpression;
import org.checkerframework.framework.qual.SubtypeOf;
import org.checkerframework.framework.qual.TypeKind;
import org.checkerframework.framework.qual.TypeUseLocation;
import org.checkerframework.framework.qual.UpperBoundFor;

/**
 * Indicates that a thread may dereference the value referred to by the annotated variable only if
 * the thread holds all the given lock expressions.
 *
 * {@code @GuardedBy({})} is the default type qualifier.
 *
 * 
The argument is a string or set of strings that indicates the expression(s) that must be held,
 * using the syntax of
 * Java expressions described in the manual. The expressions evaluate to an intrinsic (built-in,
 * synchronization) monitor or an explicit {@link java.util.concurrent.locks.Lock}. The expression
 * {@code ""} is also permitted; the type {@code @GuardedBy("") Object o} indicates that
 * the value referenced by {@code o} is guarded by the intrinsic (monitor) lock of the value
 * referenced by {@code o}.
 *
 * Two {@code @GuardedBy} annotations with different argument expressions are unrelated by
 * subtyping.
 *
 * @see Holding
 * @checker_framework.manual #lock-checker Lock Checker
 * @checker_framework.manual #lock-examples-guardedby Example use of @GuardedBy
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
@SubtypeOf(GuardedByUnknown.class)
@DefaultQualifierInHierarchy
// These are required because the default for local variables is @GuardedByUnknown, but if the local
// variable is one of these type kinds, the default should be @GuardedByUnknown.
@DefaultFor(
    value = {TypeUseLocation.EXCEPTION_PARAMETER, TypeUseLocation.UPPER_BOUND},
    typeKinds = {
      TypeKind.BOOLEAN,
      TypeKind.BYTE,
      TypeKind.CHAR,
      TypeKind.DOUBLE,
      TypeKind.FLOAT,
      TypeKind.INT,
      TypeKind.LONG,
      TypeKind.SHORT
    },
    types = {String.class, Void.class})
@UpperBoundFor(
    typeKinds = {
      TypeKind.BOOLEAN,
      TypeKind.BYTE,
      TypeKind.CHAR,
      TypeKind.DOUBLE,
      TypeKind.FLOAT,
      TypeKind.INT,
      TypeKind.LONG,
      TypeKind.SHORT
    },
    types = String.class)
public @interface GuardedBy {
  /**
   * The Java value expressions that need to be held.
   *
   * @see Syntax of
   *     Java expressions
   */
  @JavaExpression
  String[] value() default {};
}