== Why is this an issue?

In Java, value-based classes are those for which instances are final and immutable, like `String`, `Integer` and so on, and their identity relies on their value and not their reference.
When a variable of one of these types is instantiated, the JVM caches its value, and the variable is just a reference to that value.
For example, multiple `String` variables with the same value "Hello world!" will refer to the same cached string literal in memory.

The `synchronized` keyword tells the JVM to only allow the execution of the code contained in the following block to one `Thread` at a time.
This mechanism relies on the identity of the object that is being synchronized between threads, to prevent that if object X is locked, it will still be possible to lock another object Y.

It means that the JVM will fail to correctly synchronize threads on instances of the aforementioned value-based classes, for instance:

[source,java]
----
// These variables "a" and "b" will effectively reference the same object in memory
Integer a = 0;
Integer b = 0;

// This means that in the following code, the JVM could try to lock and execute
// on the variable "a" because "b" was notified to be released, as the two Integer variables
// are the same object to the JVM
void syncMethod(int x) {
    synchronized (a) {
        if (a == x) {
        // ... do something here
        }
    }
    synchronized (b) {
        if (b == x) {
        // ... do something else
        }
    }
}
----

This behavior can cause unrelated threads to deadlock with unclear stacktraces.

Within the JDK, types which should not be used for synchronization include:

* `String` literals
* Primitive wrapper classes in `java.lang` (such as `Boolean` with `Boolean.FALSE` and `Boolean.TRUE`)
* The class `java.lang.Runtime.Version`
* The ``++Optional*++`` classes in `java.util`: `Optional`, `OptionalInt`, `OptionalLong`, and `OptionalDouble`
* Various classes in the `java.time` API: `Instant`, `LocalDate`, `LocalTime`, `LocalDateTime`, `ZonedDateTime`, `ZoneId`, `OffsetTime`, `OffsetDateTime`, `ZoneOffset`, `Duration`, `Period`, `Year`, `YearMonth`, and `MonthDay`
* Various classes in the `java.time.chrono` API: `MinguoDate`, `HijrahDate`, `JapaneseDate`, and `ThaiBuddhistDate`
* The interface `java.lang.ProcessHandle` and its implementation classes
* The implementation classes of the collection factories in `java.util`: `List.of`, `List.copyOf`, `Set.of`, `Set.copyOf`, `Map.of`, `Map.copyOf`, `Map.ofEntries`, and `Map.entry`.

== How to fix it

Replace instances of value-based classes with a new object instance to synchronize on.

=== Code examples

==== Noncompliant code example

[source,java,diff-id=1,diff-type=noncompliant]
----
private static final Boolean bLock = Boolean.FALSE;
private static final Integer iLock = Integer.valueOf(0);
private static final String sLock = "LOCK";
private static final List<String> listLock = List.of("a", "b", "c", "d");

public void doSomething() {

  synchronized(bLock) {  // Noncompliant
      ...
  }
  synchronized(iLock) {  // Noncompliant
      ...
  }
  synchronized(sLock) {  // Noncompliant
      ...
  }
  synchronized(listLock) {  // Noncompliant
      ...
  }
----


==== Compliant solution

[source,java,diff-id=1,diff-type=compliant]
----
private static final Object lock1 = new Object();
private static final Integer iLock = new Integer(42);
private static final String sLock = new String("A brand new string in memory!");
private static final List<String> listLock = new ArrayList<>();

public void doSomething() {

  synchronized(lock1) { // Compliant
      ...
  }
  synchronized(iLock) { // Compliant
      ...
  }
  synchronized(sLock) { // Compliant
      ...
  }
  synchronized(listLock) { // Compliant
      ...
  }
----


== Resources

* CERT - https://wiki.sei.cmu.edu/confluence/x/1zdGBQ[Do not synchronize on objects that may be reused]
* OpenJDK - https://openjdk.java.net/jeps/390[JEP 390: Warnings for Value-Based Classes]
* Java Documentation - https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/doc-files/ValueBased.html[Value-based Classes]


ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Synchronize on a new "Object" instead.


'''

endif::env-github,rspecator-view[]
