In what scenarios does the OutOfMemoryError exception in Java occur?

As an object-oriented programming language, Java has good scalability and flexibility, but at the same time, memory overflow problems may occur. This kind of problem prompts programmers in Java through the OutOfMemoryError exception. This article will explore the occurrence scenarios of OutOfMemoryError exceptions in Java.

1. String constant pool overflow

In Java, String is a special class because it is saved in the String constant pool. When a large number of String types are used in a program, it is easy to cause the String constant pool to overflow. For example, the following code:

public static void main(String[] args) {
  List<String> list = new ArrayList<String>();
  for (int i = 0; ; i++) {
    list.add(String.valueOf(i).intern());
  }
}

This program will continuously create strings and add them to the list. Whenever a new string is created, it is checked whether the same string already exists in the String constant pool. If it already exists, a reference to the string is returned; otherwise, a new string object is created and added to the String constant pool. Since String.valueOf(i).intern() is used in the above code, all created strings will be placed in the constant pool. Therefore, when strings are continuously added to the list, new strings will be continuously created in the constant pool, eventually leading to an OutOfMemoryError exception.

2. Heap memory overflow

Java's heap memory is the heap space shared by all Java threads. When the program creates too many objects and their data structures, the heap memory may It is naturally occupied, causing an OutOfMemoryError exception. For example, the following code:

public class HeapOOM {
  static class OOMObject {
  }

  public static void main(String[] args) {
    List<OOMObject> list = new ArrayList<OOMObject>();
    while (true) {
      list.add(new OOMObject());
    }
  }
}

The above code will continuously create new OOMObject objects and add them to the list. Because OOMObject is relatively simple, each object only takes up less heap memory, so the list continues to increase and the heap memory is gradually filled up, eventually leading to an OutOfMemoryError exception.

3. Stack overflow

In Java, the storage area of ​​the stack is used for function calls when executing the program. Each function call will open up a space in the stack. Use Used to store information such as function parameters, local variables, and return values. If the function has too many call levels, it will cause stack overflow. For example, the following code:

public class StackOOM {
  public static void main(String[] args) {
    stackLeak();
  }

  private static void stackLeak() {
    stackLeak();
  }
}

In the above code, the stackLeak() method continuously calls itself recursively, because each function call will open up a space in the stack, and too many layers of calls will cause the stack space to be occupied. is full, causing an OutOfMemoryError exception.

4. Permanent generation overflow

Java's permanent generation is used to store static files, such as classes, methods, etc. Metadata storage in the JVM is placed in the permanent generation. If there are a large number of classes and methods in the application, the permanent generation will be filled up, causing an OutOfMemoryError exception. For example, the following code:

public class MetaspaceOOM {
  static class OOMObject {
  }

  public static void main(String[] args) {
    List<Class<?>> classes = new ArrayList<Class<?>>();
    while (true) {
      Class<?> c = ClassGenerator.generateClass();
      classes.add(c);
    }
  }
}

class ClassGenerator {
  public static Class<?> generateClass() {
    byte[] classData = createClassData();
    return defineClass(classData);
  }

  private static byte[] createClassData() { ... }

  private static Class<?> defineClass(byte[] classData) {
    return ClassLoader.getSystemClassLoader().defineClass(null, classData, 0, classData.length);
  }
}

The above code will continuously create new classes and add them to classes. Since the custom ClassGenerator.generateClass() method is used here to generate classes, continuously creating new classes will cause the permanent generation to be filled up, eventually causing an OutOfMemoryError exception.

Summary: OutOfMemoryError exception is a common exception in Java, usually caused by excessive memory resources consumed in the program. In actual development, if you encounter an OutOfMemoryError exception, you need to analyze it according to different scenarios and take corresponding measures to deal with it, such as increasing heap memory, optimizing code, etc.

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