Hello everyone, I'm Blackie. Today, let's talk about a headache in J**A programming - deadlocks. You've probably heard of deadlocks, or accidentally encountered them while coding. A deadlock is like a traffic jam, and in the world of a program, it can cause threads to wait endlessly, causing the program to not function properly. In j**a concurrent programming, it's critical to understand deadlocks and learn how to handle them. Next, I'm going to take you on a deep dive into deadlocks, telling you what they are, how they come about, and most importantly – how to fix them.
Let's start with what a deadlock is. To put it simply, a deadlock is a phenomenon in which two or more threads wait for each other because they compete for resources during execution, causing them to enter a stalled state. Imagine two people reaching out to grab the same chair at the same time, only to have no one catch them, but neither of them is willing to let go, and this creates an impasse. In j**a, this usually happens when multiple threads try to acquire the same lock in a different order.
Deadlocks typically occur when all four of the following conditions are met at the same time:
Mutually exclusive conditions: Resources cannot be occupied by multiple threads at the same time.
Possess and wait: A thread occupies at least one resource and waits for more resources.
Inalienable: Acquired resources cannot be forcibly taken away by other threads until they are used up.
Cycle waiting: Multiple threads form a cyclic waiting resource relationship that is connected from head to end.
Now let's look at a simple j**a deadlock example. There are two threads and two resources, each of which is needed to get the job done.
In this example, if Thread 1 locks Resource 1 and Thread 2 locks Resource 2 at the same time, they will wait for each other to release the lock, resulting in a deadlock. This is a typical scenario for deadlocks. Next, let's dive into how to avoid this from happening.
Imagine that there are two threads, one is the file writing thread and the other is the database operation thread. The file write thread needs to lock the file resource first, and then the database resource to update the state;The database operation thread, on the other hand, needs to lock the database resource first, and then lock the file resource to log it. It seems normal, but that's the trap of deadlocks.
Let's take a look at the specifics
In ** above, if Thread 1 has locked the file resource and Thread 2 has locked the database resource at the same time, then they will enter a state of waiting for each other. Thread 1 waits for thread 2 to release the database lock, and thread 2 waits for thread 1 to release the file lock, but neither can move forward. This situation is a classic scenario of deadlocks.
To avoid deadlocks, the key is to avoid at least one condition that causes deadlocks. In this example, we can avoid cyclic waits by ensuring that all threads acquire locks in the same order. For example, you can stipulate that no matter what you do, you must lock the file resource before the database resource. This way, there will be no looping waits between threads.
Preventing deadlocks may sound complicated, but with a few key strategies in place, you can greatly reduce the risk of deadlocks occurring.
The most basic rule is that locks are always acquired in a fixed order. As in the previous example, if all threads lock the file resource first and then the database resource, the deadlock will not occur. This method is simple, but very effective. Let's see how to achieve it:
Another strategy is to use lock timeouts. This means that threads don't wait indefinitely when trying to acquire locks. j**areentrantlockSuch a feature is provided. Let's look at an example:
This method works by trying to acquire a lock, and if it fails, releasing the lock it already holds, and then trying again later. This reduces the risk of threads permanently hanging due to deadlocks.
Finally, the J**A concurrency API provides some advanced tools, such as:j**a.util.concurrentThe classes in the package can help us better manage locks and avoid deadlocks. For example,semaphorecan be used to control the number of concurrent access to resourcescountdownlatchwithcyclicbarrierIt can be used for synchronization between threads.
Let's talk about how to detect and solve the deadlock problem in J**A. As your program gets bigger and threads increase, deadlocks become more unavoidable. Fortunately, there are tools and tricks that can help us identify and resolve these tricky deadlocks.
5.1 Use the JVM tool to detect deadlocks.
The J**A Virtual Machine (JVM) provides some built-in tools to help detect deadlocks, such as JWoweb and JVontexTM. These tools allow you to see the status of your threads to see if there are deadlocks.
For example, when using JWover, you can simply connect to your JAy application and look at the "Threads" tab. If there is a deadlock, the tool will alert you and show you which threads and resources are deadlocked.
5.2 Programming tips to solve deadlocks.
Once you know that deadlocks exist, solving them is the next challenge. If the deadlock is due to an inappropriate lock order, readjusting the order in which the locks are acquired is a simple and effective solution. But in more complex cases, more detailed investigation and modification may be required.
5.3 Precautionary measures.
Prevention is better than repair, so it's important to write your ** with deadlocks in mind. Keep it simple to avoid a thread holding multiple locks at the same time, and if needed, use a timeout to try to acquire a lock, which allows the thread to give up or retry if the lock waits too long.
This paragraph shows simple lock guards. By ensuring that all threads follow the same order in which locks are acquired, deadlocks can be effectively prevented.
Detecting and resolving deadlocks is a complex process that requires patience and meticulous investigation. But as long as you understand how deadlocks work and follow best practices, you can effectively reduce the occurrence of deadlocks.
After the first few chapters, we have learned a lot about deadlocks. Now, let's summarize the best practices for avoiding and handling deadlocks in concurrent programming to ensure that your J**A application runs more smoothly and efficiently.
6.1 Summary of best practices.
Keep locks simple: Try to avoid nesting multiple locks, which reduces the likelihood of deadlocks.
Lock order consistency: Locks are always acquired in the same order, which prevents cyclic waits from occurring.
Use timed locks: Use trylock with timeout to prevent threads from blocking for a long time.
Avoid unnecessary locks: Analyze** to ensure that locks are only applied when necessary.
Use advanced concurrency tools, such as reentrantlock, semaphore, etc., which provide more complex lock operations to help solve complex concurrency problems.
Review & Test: Conduct regular reviews to look for potential deadlock risks, while conducting thorough multi-threaded testing.
6.2 An example of deadlock resolution.
Let's demonstrate the application of these best practices with a simple example:
This example uses:reentrantlockand timeout attempts to acquire locks, effectively avoiding deadlocks.
Deadlocks are a common problem in concurrent programming, but by following some basic principles and best practices, we can effectively reduce and solve this problem. Remember, a good programmer isn't just the one who writes, it's the guardian who makes sure it's robust and efficient. Hopefully, this blog will be helpful to you on your j**a concurrent programming journey!
Well, that's all for today's sharing. See you next time as we continue to delve into more of the mysteries of JA programming!
Author: Song Xiaohei.
Link: juejincn/post/7308219781055528998