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checker-qual contains annotations (type qualifiers) that a programmer writes to specify Java code for type-checking by the Checker Framework.

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package org.checkerframework.checker.lock.qual;

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;

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;

/**
 * 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 {};
}