Java OOM Exception scenarios and troubleshooting processes (heap, stack, method area)

Java OOM exception scenarios and troubleshooting (heap, stack, method area)

1. Introduction

During Java application development and running, the OutOfMemoryError (OOM) exception is a common and headache-inducing problem.

OOM exception indicates that a Java virtual machine (JVM) cannot meet the needs when trying to allocate more memory, which usually means that there is a problem with the program's memory usage. According to the different memory areas, OOM exceptions can be mainly divided into heap memory overflow, stack memory overflow and method area memory overflow.

Below we will discuss these abnormal scenarios in detail and the corresponding troubleshooting methods.

2. Heap memory overflow (Heap Space)

2.1 Exception Scenario

The heap is the area used in the Java virtual machine to store object instances. When the program continuously creates new objects and these objects are referenced and cannot be garbage collected, the heap memory will be continuously occupied, which will eventually lead to heap memory overflow. Common situations include:

Memory leak: Objects are no longer used, but due to program design issues, these objects are still referenced and cannot be garbage collected. For example, an object was added to the collection, but the object that was no longer used was not removed correctly.
Large object creation: Very large objects are created in the program, such as large arrays, large collections, etc., which exceed the available space in heap memory.
Too many objects: The program creates a large number of objects in a short time, which makes the heap memory unacceptable.

2.2 Sample code

import ;
import ;

public class HeapOOMExample {
    public static void main(String[] args) {
        List&lt;byte[]&gt; list = new ArrayList&lt;&gt;();
        while (true) {
            (new byte[1024 * 1024]); // Create 1MB array each time        }
    }
}

2.3 Troubleshooting methods

Use tools to monitor heap memory usage: You can use VisualVM, Java Mission Control and other tools to monitor the usage of heap memory and view the growth trend of heap memory, the distribution of objects and other information.
Analyze heap dump files: Added at JVM startup-XX:+HeapDumpOnOutOfMemoryErrorParameters: When a heap memory overflow occurs, the JVM will automatically generate a heap dump file (.hprof). Then use a tool (such as Eclipse Memory Analyzer) to analyze the heap dump file and find objects that occupy a lot of memory.

3. Stack memory overflow ()

3.1 Exception Scenario

The Java stack is an area used to store information such as local variables, operand stacks, dynamic links, etc. of method calls. Each thread has its own independent stack space. Stack memory overflow is usually caused by the depth of the method call, which causes the stack frames to be continuously put into the stack, and eventually exhausts the stack space. Common situations include:

Recursive call has no termination condition: When there is no correct termination condition, the recursive method will constantly call itself, resulting in an infinite increase in stack frames.
The method call chain is too long: There are complex method call chains in the program, and each method creates a stack frame. When the call chain is too long, the stack space will be exhausted.

3.2 Sample code

public class *Example {
    public static void recursiveMethod() {
        recursiveMethod(); // Infinite recursive call    }

    public static void main(String[] args) {
        recursiveMethod();
    }
}

3.3 Troubleshooting methods

View exception stack information: When the stack memory overflows, the JVM will throw it*ErrorException and output detailed exception stack information. By analyzing the stack information, you can locate the method that has problems.
Reduce recursive depth or optimize method call chains: Check whether the recursive method has the correct termination condition, or consider using iterative instead of recursion. For complex method call chains, optimization can be performed to reduce unnecessary method calls.

4. Metaspace memory overflow

4.1 Exception Scenario

The method area is mainly used to store the metadata information of the class, such as class definition, constant pool, method bytecode, etc. In Java 8 and later versions, the method area is implemented by metaspace. Memory overflow in the method area is usually caused by the program dynamically generating a large number of classes, which makes the metaspace unable to accommodate the metadata information of these classes. Common situations include:

Dynamic proxy frequently used: Using dynamic proxy technology will generate new proxy classes at runtime. When dynamic proxy is frequently used, a large number of proxy classes will be generated, occupying metaspace.
Loading classes in large quantities: In some frameworks (such as Spring, Hibernate, etc.), a large number of classes will be loaded dynamically. If there is no reasonable class loading mechanism, it will cause metaspace memory to overflow.

4.2 Sample code

import ;

public class MetaspaceOOMExample {
    public static void main(String[] args) {
        while (true) {
            (
                    (),
                    new Class<?>[]{},
                    (proxy, method, args1) -> null
            );
        }
    }
}

4.3 Troubleshooting methods

Monitor metaspace usage: Use tools such as VisualVM to monitor the usage of metaspace and view the growth trend of metaspace.
Adjust the metaspace size: When the JVM is started, it can be passed-XX:MetaspaceSizeand-XX:MaxMetaspaceSizeParameters to adjust the initial size and maximum size of the metaspace.
Check the class loading mechanism: Ensure that there are no unnecessary class loading operations in the program and avoid dynamically generating too many classes.

Summarize

Java OOM exceptions are a complex problem. The causes and troubleshooting methods for OOM exceptions in different memory areas are also different. During the development and operation and maintenance process, it is necessary to pay close attention to the memory usage of the program, reasonably adjust JVM parameters, and optimize code logic to avoid the occurrence of OOM exceptions. When an OOM exception occurs, appropriate troubleshooting methods should be adopted according to the type and specific situation of the exception to find out the root cause of the problem and solve it.

The above is personal experience. I hope you can give you a reference and I hope you can support me more.