Shulou(Shulou.com)06/01 Report--

Today, I will talk to you about how to detect and avoid deadlocks in Java. Many people may not know much about it. In order to make you understand better, the editor has summarized the following for you. I hope you can get something according to this article.

If you are not involved in the coding of multithreaded concurrent Java applications, you may fail.

# how to avoid Java thread deadlock?

This is one of the hot questions in Java interviews and one of the most important tastes in multithreaded programming. It is easy to be asked when recruiting senior programmers, and there are a lot of follow-up questions.

Although the problem seems very basic, most Java developers will get into trouble once you start digging into it.

"what is a deadlock?

When two or more threads are waiting for each other to release the required resources (lock) and fall into an infinite wait is a deadlock. It occurs only in the case of multitasking or multithreading.

# how to detect deadlocks in Java?

Although there are many answers to this, my version is that first I look at the code, if I see a nested synchronization block, or call other synchronization methods from a synchronized method, or try to acquire locks on different objects, if developers are not very careful, it is easy to cause deadlocks.

Another way is to find the application when it is actually locked and try to take a thread dump. In Linux, you can do this through the kill-3 command, follow the official account duckling programming community, reply to the keyword manual, and get the latest development manual, which will print the status of all threads in the application log file, and you can see which thread is locked on which thread object.

You can use tools such as the fastthread.io website to analyze the thread dump, which allows you to upload the thread dump and analyze it.

Another way is to use jConsole or VisualVM, which will show which threads are locked and which objects are locked.

If you are interested in learning about troubleshooting tools and analyzing thread dumps, I suggest you take a look at Uriah Levy's "analyzing Java thread dumps" course on multiple Vision (PluraIsight). Designed to learn more about Java thread dumps and to be familiar with other popular advanced troubleshooting tools.

# write a Java program that will lead to deadlocks?

Once you have answered the previous question, they may ask you to write code, which will lead to a Java deadlock.

/ * the Java program creates a deadlock by forcing a loop to wait. * / public class DeadLockDemo {

/ * * this method requests two locks, the first string, and then the integer * / public void method1 () {synchronized (String.class) {System.out.println ("Aquired lock on String.class object")

Synchronized (Integer.class) {System.out.println ("Aquired lock on Integer.class object");}

/ * * this method also requests the same two locks, but in exactly the opposite order, that is, the integer first, and then the string. * if one thread holds a string lock then this will result in a potential deadlock * and others hold integer locks that they wait for each other forever. * / public void method2 () {synchronized (Integer.class) {System.out.println ("Aquired lock on Integer.class object")

Synchronized (String.class) {System.out.println ("Aquired lock on String.class object");}}

If both method1 () and method2 () are called by two or more threads, there is a possibility of deadlock, because if thread 1 acquires a lock on the Sting object while executing method1 (), thread 2 acquires the lock on the Integer object while executing method2 (), waiting for each other to release the locks on Integer and String to continue with one step, but this will never happen.

This figure accurately demonstrates our program, where one thread holds locks on one object and waits for other object locks held by other threads.

As you can see, Thread1 needs the lock on Object2 held by Thread2, while Thread2 wants to get the lock on Object1 held by Thread1. Because no thread is willing to give up, there is a deadlock and the Java program is stuck.

The idea is that you should know the right way to use common concurrency patterns, and if you are not familiar with these patterns, then Jose Paumard's "common Java patterns applied to concurrency and multithreading" is a good starting point for learning.

# how to avoid deadlocks in Java?

Now the interviewer comes to the last part, in my opinion, one of the most important parts; how to fix the deadlock in the code? Or how to avoid deadlocks in Java?

If you take a closer look at the above code, you may have found that the real cause of the deadlock is not multiple threads, but the way they request the lock, and if you provide orderly access, the problem will be resolved.

Here is my fix version, which avoids deadlocks without preemption by avoiding loop waits, which is one of the four conditions that require deadlocks.

Public class DeadLockFixed {

Both methods now request locks in the same order, first with integers and then with String. * you can also do the reverse, for example, the first string, and then the integer, * as long as both methods request locking, both can solve the problem * in the same order. * / public void method1 () {synchronized (Integer.class) {System.out.println ("Aquired lock on Integer.class object")

Synchronized (String.class) {System.out.println ("Aquired lock on String.class object");}

Public void method2 () {synchronized (Integer.class) {System.out.println ("Aquired lock on Integer.class object")

Synchronized (String.class) {System.out.println ("Aquired lock on String.class object");}}

There are no deadlocks now because both methods access locks on the text of the Integer and String classes in the same order.

Therefore, if thread An acquires a lock on the Integer object, thread B will not continue until thread A releases the Integer lock, and even if thread B holds the String lock, thread A will not be blocked, because now thread B does not expect thread A to release the Integer lock to continue.

After reading the above, do you have any further understanding of how to detect and avoid deadlocks in Java? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

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