The method to judge Java memory leaks includes observing the continuous growth of heap memory, frequent Full GC and poor recycling effect, OutOfMemoryError exceptions, and can be analyzed by jstat and jmap. 2. Common reasons include cache not being cleaned, listener not being logged out, ThreadLocal not being cleaned, and static collection abuse. The response methods are to use weak references or regular cleaning, timely anti-registration, call remove(), and reasonably design static collection cleaning logic. 3. In terms of tools, it is recommended to assist in positioning leakage points such as VisualVM, Eclipse MAT, YourKit, etc., and you can observe the growth trend of the object by comparing heap dump. 4. After repair, it is necessary to simulate the load and perform stress tests in the test environment. After it is launched, the GC log and memory changes are continuously monitored to confirm stability.

Although Java memory leaks are not as conspicuous as C, once they occur, they will affect performance at the least, and at the worst, they will cause system crashes. This type of problem is usually not easy to detect, especially in online environments, which is also troublesome to troubleshoot. The key is to detect and deal with it early.

1. How to determine whether there is a memory leak in a Java application?

To determine whether a memory leak occurs, we must first look at the behavior of GC and the changing trend of heap memory.

• Continuously growing heap memory usage : If the memory of the old generation does not drop significantly after each Full GC, and the overall heap usage continues to rise, it is likely to be a memory leak.
• Frequent Full GC : JVM frequently triggers Full GC, but the recycling effect is very small, which also means that some objects have been retained and cannot be released.
• OutOfMemoryError Exception log : java.lang.OutOfMemoryError: Java heap space or GC overhead limit exceeded errors, which is an obvious signal.

At this time, you can first use jstat -gc to view the GC situation, and then use jmap -histo to see which class instances occupy a lot of memory.

2. Common causes of memory leaks and how to deal with them

Most Java memory leaks are "unconscious object retention", which means that the object is no longer used, but is still referenced, resulting in the GC being unable to be recycled.

Several common situations include:

• Cache not cleared : For example, HashMap is used for cache but no expiration mechanism is added.
• Listeners and callbacks are not logged out : For example, the event listener is not cancelled in time after registration.
• ThreadLocal is not cleared : especially when using thread pools, thread reuse can easily cause memory backlog.
• Abuse of static collection classes : Static variables have a long life cycle and are easily "trash can".

The solution is not complicated:

• Consider using weak references (WeakHashMap) or regular cleaning mechanisms for caches.
• Remember to reverse register after using listeners, callbacks, etc.
• Remember to call remove() method after ThreadLocal is used up.
• Avoid unnecessary static collections, and design the cleaning logic if you really need it.

3. Tool-assisted positioning of the source of the leak

It is inefficient to rely solely on code self-checking, and it is recommended to analyze it in combination with tools.

Commonly used tools are:

• VisualVM : A graphical interface, which can directly connect to local or remote JVMs to view heap memory, threads, class loading, etc., and can also do heap dump analysis.
• Eclipse MAT (Memory Analyzer) : It is specially used to analyze heap dump files and find large objects and potential leak points in the "dominant tree".
• YourKit/JProfiler : A commercial tool with more powerful functions and is suitable for in-depth analysis in complex scenarios.
• Command line tools such as jmap jhat : suitable for situations where there is no graphical environment. You can generate a heap dump through jmap -dump , and then open the analysis with jhat or MAT.

A practical technique is: compare the heap dumps at different time points several times and observe which class instances continue to increase, which is basically a suspicious point.

4. Details to be paid attention to during actual repair

To fix memory leaks, you cannot just rely on changing the code, but also pay attention to verification after it is launched.

• Test environment simulates online load : try to restore the real scene as much as possible to expose the problem.
• Do stress test monitoring before going online : Ensure that the modification is effective and no new problems are introduced.
• Long-term observation of GC logs and memory changes : confirm that the memory tends to stabilize after repair.

For example, a project used Spring's ApplicationListener before, but a certain listening class was not destroyed in time, resulting in the entire bean being unable to be recycled. Finally, the reference chain of the listener was found through MAT, and the @PreDestroy annotation was added to clean it up, and the problem was solved.

Basically that's it. Memory leaks are not particularly profound. The key is to consciously avoid "invisible references" when writing codes. When problems arise, you can quickly use tools to locate the root cause.

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