16f6c0aecf
Inline adoc files when they are included exactly once. Also fix language tags because this inlining gives us better information on what language the code is written in.
58 lines
2.0 KiB
Plaintext
58 lines
2.0 KiB
Plaintext
== Why is this an issue?
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According to the documentation,
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____
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A program may produce unpredictable results if it attempts to distinguish two references to equal values of a value-based class, whether directly via reference equality or indirectly via an appeal to synchronization...
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____
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This is because value-based classes are intended to be wrappers for value types, which will be primitive-like collections of data (similar to ``++struct++``s in other languages) that will come in future versions of Java.
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____
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Instances of a value-based class ...
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* do not have accessible constructors, but are instead instantiated through factory methods which make no commitment as to the identity of returned instances;
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____
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This means that you can't be sure you're the only one trying to lock on any given instance of a value-based class, opening your code up to contention and deadlock issues.
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Under Java 8 breaking this rule may not actually break your code, but there are no guarantees of the behavior beyond that.
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This rule raises an issue when a known value-based class is used for synchronization. That includes all the classes in the ``++java.time++`` package except ``++Clock++``; the date classes for alternate calendars, ``++HijrahDate++``, ``++JapaneseDate++``, ``++MinguoDate++``, ``++ThaiBuddhistDate++``; and the optional classes: ``++Optional++``, ``++OptionalDouble++``, ``++OptionalLong++``, ``++OptionalInt++``.
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*Note* that this rule is automatically disabled when the project's ``++sonar.java.source++`` is lower than ``++8++``.
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=== Noncompliant code example
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[source,java]
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----
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Optional<Foo> fOpt = doSomething();
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synchronized (fOpt) { // Noncompliant
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// ...
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}
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----
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== Resources
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* https://docs.oracle.com/javase/8/docs/api/java/lang/doc-files/ValueBased.html[Value-based classes]
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ifdef::env-github,rspecator-view[]
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'''
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== Implementation Specification
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(visible only on this page)
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=== Message
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Synchronize on a non-value-based object; synchronizing on a "xxx" could lead to contention.
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endif::env-github,rspecator-view[]
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