To prevent this from happening, the Java compiler generates matching enter and exit instructions in such a way that once execution has entered into a synchronized block or method, it must pass through a matching MonitorExit instruction for the same object. It’s worth noting that since the lock is reentrant, the thread owning the lock may continue to happily execute even if it were to reach and reenter the same lock again.Īnd here’s the catch.
In this case the thread owning the lock can cause other threads who are trying to obtain the lock to block indefinitely. The result of such a case would be that the MonitorExit instruction with throw an IllegalMonitorStateException.Ī more dangerous case is what would happen if a lock is acquired via MonitorEnter, but isn’t released via a corresponding call to a MonitorExit.
FILE SYNCHRONIZATION JAVA CODE
Even so, from the JVM’s perspective such code is totally valid. So what happens if one of the two instructions is called without a respective call to the other? The Java compiler will not produce code that calls MonitorExit without calling MonitorEnter. The object to lock is also loaded from the operand stack, previously placed there by either dereferencing a variable, field or invoking a method returning an object. This is unusual, as most bytecode instructions are independent of each other, usually “communicating” with one another by placing values on the thread’s operand stack. There are two bytecode instructions responsible for synchronization. To avoid these, it’s recommended to use a privately held object as a lock to prevent accidental sharing or escapement of locks. This creates implicit synchronization dependencies across the hierarchy and has the potential of creating throughput issues or even deadlocks. A worse scenario is declaring synchronized methods in a base class (which might even be a 3rd party class) and then adding new synchronized methods to a derived class. One example is creating implicit dependencies between different synchronized methods of the same object, as they share the same lock. Synchronized methods can sometimes cause bad behavior. For static methods the lock will be placed on the Class object. For synchronized methods the lock is automatically selected to be the “this” variable.
These instructions operate on an object specified explicitly by the developer in the context of the synchronized block. This differs from other locking mechanisms, such as those found in the package, which are implemented (in the case of HotSpot) using a combination of Java code and native calls made through. Synchronized code blocks are implemented using two dedicated bytecode instructions, which are part of the official specification – MonitorEnter and MonitorExit.
The synchronized keyword is used in two primary contexts: It is also at the base of many of the more complex patterns we use such as thread and connection pools, concurrent collections and more. This mechanism powers the synchronized keyword, making it one of, if not the most popular multi-threading idiom in Java. To better understand these, let’s explore the most synchronization idiom – the Object lock. These include object pools, concurrent collections, advanced locks, execution contexts etc. Java and Scala provide a multitude of components to write solid multi-threaded applications. Most of the synchronization work is done for us at the framework level, such as by our web server, DB client or messaging framework. Practically all server applications require some sort of synchronization between multiple threads.
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