Question
Answer and Explanation
A StackOverflowError in WildFly (or any Java application server) typically arises from a recursive method call that doesn't have a proper base case, or from a deep call stack that exceeds the allocated stack space. Here's a breakdown of why this happens and how to troubleshoot it:
Understanding StackOverflowError:
In Java, each thread has its own stack, which is a region of memory used to store method calls and their local variables. When a method calls another method, the details of the call are added to the stack. If a method keeps calling itself (directly or indirectly) without a way to stop, this "stack" will eventually overflow, resulting in a StackOverflowError. The Java Virtual Machine (JVM) doesn't know when to stop in such case. This causes the application to crash and terminate the processing of this thread.
Common Causes on WildFly:
1. Infinite Recursion in Application Code:
- The most common reason is a recursive method that doesn't have a proper termination condition. For example:
public int badRecursiveMethod(int number) {
return badRecursiveMethod(number + 1); // No base case!
}
This method will keep calling itself, eventually filling up the stack and causing the error.
2. Deep Call Chains:
- While not always a recursive issue, excessively deep method call chains (one method calls another, which calls another, etc.) can also consume stack space. This may occur in complex applications with deep data structures and processing logic.
3. Issue in Library Code:
- Sometimes, the recursion can exist in the libraries used by your application, or even in WildFly internals. However, such cases are less frequent.
4. Large Object Allocation on Stack:
- While less common, if you're allocating a very large local variable or array within a method, this can contribute to stack overflow, especially if such a method is part of a recursion, or in a deeply nested call stack.
Troubleshooting Steps:
1. Examine the Stack Trace:
- The stack trace will point to the sequence of method calls that led to the error. Look for repeating method calls, which might indicate recursion or deep call chains.
2. Review Recursive Functions:
- Carefully examine your recursive functions. Ensure that they have a base case that stops the recursion. A base case is when a function doesn't call itself, but it returns a specific value based on the input provided.
3. Simplify Code:
- If the issue is due to deep call chains, refactor or simplify your code to reduce the depth of method calls.
4. Increase Stack Size (Last Resort):
- You can increase the stack size for the JVM using the -Xss JVM option. For example, -Xss2m would allocate 2 megabytes of stack space. However, this is typically only a workaround, as increasing the stack size won’t fix underlying problems with your code. It should be used with caution and should only be considered when you have a good understanding of your application behavior and the stack usage. This might help, if the StackOverflow is not caused by recursive call, and the problem is complex call chain.
To set this option for WildFly, modify the relevant configuration file (e.g., standalone.conf or domain.conf depending on your setup) to include:
JAVA_OPTS="$JAVA_OPTS -Xss2m"
5. Use Debugging Tools:
- Employ a Java debugger (such as the one provided by your IDE) to step through the code and observe the execution flow, paying close attention to recursive calls and stack frames. Tools can help you identify the problem and location where the crash happens. You might be able to find the problem that you wouldn't find otherwise.
Important Note: Increasing the stack size should be a last resort. It’s almost always better to address the underlying code issues (recursion or deep call chains). Using -Xss option with big number can consume a lot of memory and can have the negative impact on your application performance. So first, make sure that your application code is well written, and that the StackOverflowError is not caused by an infinite recursive function.
By following these debugging steps, you should be able to pinpoint the cause of your StackOverflowError and prevent it from occurring in the future.