How to implement Java underlying technology memory allocation and garbage collection

How to implement Java's underlying technology of memory allocation and garbage collection

In Java, memory allocation and garbage collection are very important underlying technologies. Correctly understanding and mastering memory allocation and garbage collection technology can effectively improve program performance and resource utilization. This article will introduce the principles and implementation methods of memory allocation and garbage collection in Java, and give relevant code examples.

1. Memory allocation
Memory allocation in Java is responsible for the Java Virtual Machine (JVM). The JVM divides the memory area into several different parts, including heap, stack, method area, etc. The specific memory allocation process is as follows:

1. Heap memory: Heap memory is the most important memory area in a Java program and is used to store all object instances. Heap memory is dynamically allocated. When the program needs to create a new object, the JVM will find a sufficient free area in the heap memory to store the object's instance variables, and then allocate memory for the object in the heap.

2. Stack memory: Stack memory is used to store local variables and temporary data when calling methods. Each thread has its own stack memory. When the thread executes a method, the parameters and local variables of the method are stored in the stack memory. The allocation and release of stack memory occurs automatically. When a method is executed, the stack frame will be destroyed immediately.

3. Method area: The method area is used to store class information, static variables, constant pools, etc. The memory allocation of the method area is completed when the program starts and will not be dynamically expanded while the program is running.

The following is a simple code example showing the memory allocation process in Java:

public class MemoryAllocationExample {
  public static void main(String[] args) {
    // 创建一个对象实例
    ExampleObject obj = new ExampleObject();
  }
}

class ExampleObject {
  private int value;

  public ExampleObject() {
    value = 10;
  }
}

In the above example, when executing ExampleObject obj = new ExampleObject(); statement, the JVM will allocate memory space for the ExampleObject object in the heap memory, and call the object's constructor to initialize the instance variables.

2. Garbage Collection
In Java, garbage collection (Garbage Collection) is performed automatically. The JVM will periodically scan the heap memory to check which objects are no longer referenced, and mark the memory space occupied by these objects as recyclable. The specific process of garbage collection is as follows:

1. Marking phase: The JVM will start traversing from the root object (such as static variables, classes in the method area) and mark all referenced objects.

2. Clearing phase: JVM will clear all unmarked objects and release the memory space they occupy.

3. Compression phase: After the cleanup phase is completed, the JVM may organize the surviving objects in order to better utilize the memory space.

The following is a simple code example that shows the garbage collection process in Java:

public class GarbageCollectionExample {
  public static void main(String[] args) {
    // 创建一个对象实例
    ExampleObject obj = new ExampleObject();

    // 将obj赋值为null，表示不再引用该对象
    obj = null;

    // 执行垃圾回收
    System.gc();
  }
}

class ExampleObject {
  private int value;

  public ExampleObject() {
    value = 10;
  }

  @Override
  protected void finalize() throws Throwable {
    // 垃圾回收时调用的方法
    System.out.println("Garbage collection triggered.");
  }
}

In the above example, when the System.gc() statement is executed , the JVM will perform garbage collection operations. During garbage collection, the finalize() method of the ExampleObject object will be called and a message will be output.

Summary: Correctly understanding and using Java's underlying technology, memory allocation and garbage collection, is very important for writing high-performance and high-reliability Java programs. Through in-depth study and practice, we can better grasp the principles and implementation methods of Java memory allocation and garbage collection, and then optimize and improve our applications.

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