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The [[Java]] synchronization model with all its '''synchronized''', '''notify''' and '''wait''' is commonly thought of as  an example of [[monitor]]s. However with closer look one finds out that it is far different to the model used in [[wikipedia::Concurrent_Pascal|Concurrent Pascal]]. It is not that robust, it is not well enough integrated to the language and sort of feels like an assembly language with synchronization primitives than high-level abstraction provided by real [[monitor]]. However, it has yet another hidden flaw, kind of example of [[Copy Based Design]] problem: the [[Java Monitor]]s implant the [[monitor]] into [[wikipedia::Object_oriented_language|object oriented language]], in an object oriented style and that does not work.

Imagine there is an '''Cache''' support class in some [[API]]:

<source lang="java" snippet="monitor.pitfalls.Cache"/>

Its synchronization is sufficiently well designed. It never calls foreign code while holding own lock, which is one of necessary [[Deadlock Condition]]s. As such one would expect that there is no way to [[deadlock]] on the '''Cache''''s own lock. However that would be false assumption. Simple call like:

<source lang="java" snippet="monitor.pitfalls.brokencall"/>

can lead to [[deadlock]] while waiting to enter internal(!) '''Cache''''s [[monitor]]:

<source lang="text">
Thread Test Watch Dog: testDeadlockViaAPI
 org.apidesign.javamonitorflaws.Cache.inspectValue:32
 org.apidesign.javamonitorflaws.Cache.get:22
 org.apidesign.javamonitorflaws.CacheTest.testDeadlockViaAPI:134
</source>

Moreover this all is achievable using the ''best'' [[Java]] synchronization practices. How's that possible? 

Just imagine there is a subclass of the '''Cache''' class which defines few properties and for better or worse guards them as [[Java Monitor]]: 

<source lang="java" snippet="monitor.pitfalls.subclass"/>

This code is not bulletproof. It notifies its listeners while holding own lock and as such this can lead to deadlocks. However it is common that people do these sort of mistakes when using an [[API]]. The [[API]] shall be ready to handle such [[cluelessness|clueless]] clients. It is acceptable to let them deadlock on their own locks, yet there is no reason to block the [[API]] itself. It shall continue to work for other clients if possible. Yet this is the case in this example:

<source lang="java" snippet="monitor.pitfalls.block.propertychange"/>

Simple call to '''get("123")''' deadlocks. Why? Because the [[Java Monitor]]s  implant the [[monitor]] concept into object oriented world and in the process of doing so break the most fundamental assumption: the [[monitor]] shall be encapsulated. 

One shall always define all internal data structures and all their operations working on them in one monitor. This is not the case of [[Java]]'s synchronized methods at all. By default, the methods in '''Cache''' and its subclass '''MultiplyCache''' share the same [[monitor]] lock!

=== Implications for [[API]] Design ===

Now, after blaming [[Java]] of having wrong synchronization model, it is our turn to explain what implications does this have for [[API]] design. Are the above described problems reason why one cannot write good [[API]] in [[Java]]? No, not at all. One just cannot do it [[Cluelessness|cluelessly]], one cannot use the most obvious tool - '''synchronized''' methods - when writing a shared library.

Exposing locks in an [[API]] has its own obligations. Other people can obtain them and the [[API]] has to know what to do then. Having some global, publicly accessible locks exposed to unknown clients is a way to ask for problems with [[deadlock]]s. 

Using ''synchronized'' methods in classes makes the locks accessible to [[API]] clients for public use. Moreover this often happens unconsciously, which is even more dangerous. The fix is simple. Just hide the locks as private objects:

<source lang="java" snippet="monitor.pitfalls.CacheOK"/>

With a change like this the call

<source lang="java" snippet="monitor.pitfalls.brokencall"/>

can never [[deadlock]] again, regardless how bad or clueless [[API]] clients one has. 

=== The way out ===

{{:Synchronized}}

[[Category:APIDesignPatterns]]
[[Category:APIDesignPatterns:Anti]]

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