圜rash provides an insightful transitive dependency graph of 'BLOCKED' threads, aiding in understanding the thread contention. Tools like fastThread and 圜rash are invaluable for this purpose. To diagnose such issues non-intrusively, the recommended approach is thread dump analysis. When threads enter the 'BLOCKED' state, it's essential to identify the specific lines of code or methods causing this behavior. The thread that successfully acquires the lock will proceed to execute the locked critical section. Once the lock is released by the owning thread (the thread that currently holds the lock), one of the blocked threads will be chosen to acquire the lock and transition from the ‘BLOCKED’ state to the ‘RUNNABLE’ state. Instead, it will go into the ‘BLOCKED’ state and wait until the lock becomes available. ReentrantLock: If a thread attempts to acquire a ReentrantLock that is already held by another thread, it will not immediately get the lock. If a thread tries to enter a synchronized block that is already locked by another thread, it goes into the ‘BLOCKED’ state and waits for the lock to be released.ģ. Synchronized block: When multiple threads contend for access to a synchronized block, only one thread can execute the block at a time. In general it's a good practice to avoid synchronized methods as much as possible for better performance.Ģ. If this synchronized method happens to be in a critical code path, then there is a high possibility for multiple threads to enter into this ‘BLOCKED’ state. If any other tries to invoke this ‘synchronized’ method when some other is already executing this method, it will be put into the ‘BLOCKED’ state. When a method is marked with 'synchronized' keyword, JVM will allow only one thread to enter into that ‘synchronized’ method at any given time. Here the 'getData()' method is defined as a 'synchronized' method. Synchronized method: Threads enter the ‘BLOCKED’ state when they contend for access to synchronized method. When will a thread enter into a BLOCKED state?Ī Thread will enter in to ‘BLOCKED’ state in one of the following scenarios:ġ. In this state, the blocked thread cannot proceed until the resource it needs becomes available. BLOCKEDĪ thread enters the ‘BLOCKED’ state when it's trying to access a resource that's currently held by another thread. Let's discuss these 3 states with real-life fun examples :). When a thread is suspended (i.e., unable to progress further), it will be one of these 3 suspended thread states: BLOCKED, WAITING, TIMED_WAITING. The only way one may ensure that the second thread will always read the value written by the first thread is by the means of synchronization, or by declaring the counter volatile, but we will not cover the volatile modifier in this tutorial.Java threads can be in 6 different states: NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, TERMINATED. This behaviour is due to the Java Memory Model specification. If the methods were not synchronized and a given thread increments the counter, when a second thread reads the counter value it is not guaranteed that the second thread will read the value that was written by the first thread, even if the write happens before the read at wall clock time. If a given thread acquires the lock and increments the counter, any other thread that subsequently acquires the lock and reads the counter will read the value that was written by the previous thread. Synchronizing static methodsīy synchronizing both increment and read methods we are establishing a happens-before relationship between the two methods. Since they are synchronized on distinct object's intrinsic locks they will stay waiting for each other for undetermined time. We also know that our someMethod will call other's doSomething method. We know that doSomething method will call our secondMethod. Meanwhile Thread2 happens to hold a reference to the same other instance and calls its doSomething method, acquiring the other intrinsic lock. Thread1 creates an instance of our object and calls its someMethod acquiring our intrinsic lock due to the synchronized statement. Supposing that other's object doSomething method is also synchronized on the other object intrinsic lock, and that the same doSomething method in turn calls our object's secondMethod, in a scenario where multiple threads are executing methods on these two instances we could reach a deadlock situation.
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