Java 中的类内存

10 Sept 2024 | 4 分钟阅读

类在内存中的结构

Java 程序中的每一个类在编译时都会被转换成字节码。字节码的主要目的是存储将由 Java 虚拟机 (JVM) 执行的指令。Class 对象负责包含有关类结构、方法、字段和各种元数据的所有信息和数据。

类加载

Java 采用动态类加载机制。JVM 会加载 Java 程序运行时所需和使用的所有类。所有类文件都由类加载器子系统查找和加载。有各种可用的类加载器，例如 Bootstrap 类加载器、Extension 类加载器等。所有类型的类加载器协同工作，将类加载到分配给 Java 虚拟机 (JVM) 的内存中。

让我们通过几个 Java 示例程序来详细了解 Java 中的类内存。

示例 1

文件名: ClassMemory.java

// Java program for demonstrating the features and functionalities of Class memory in Java
class ParentClass {
    private String data;
    public ParentClass(String data) {
        this.data = data;
    }
    public void classTask() {
        // Simulating a complex task with additional data
        for (int i = 0; i < 100; i++) {
            this.data += i;
        }
    }
}
public class ClassMemory {
    private int int_Value;
    private String string_Value;
    private ParentClass functionality;
    public ClassMemory(int int_Value, String string_Value) {
        this.int_Value = int_Value;
        this.string_Value = string_Value;
        this.functionality = new ParentClass("AdditionalData");
    }
    public void display_Memory_Usage() {
        long total_Memory = Runtime.getRuntime().total_Memory();
        long free_Memory = Runtime.getRuntime().free_Memory();
        long used_Memory = total_Memory - free_Memory;
        System.out.println("Total Memory Allocated: " + total_Memory + " bytes");
        System.out.println("Free Memory Allocated: " + free_Memory + " bytes");
        System.out.println("Used Memory Allocated: " + used_Memory + " bytes");
        System.out.println("------------------------");
    }
    public void additional_Task() {
        for (int i = 0; i < 100; i++) {
            this.int_Value += i;
            this.string_Value += "X";
        }
        this.functionality.classTask();
    }
    public static void main(String[] args) {
        // Creation of an array of objects
        ClassMemory[] object_Array = new ClassMemory[1000];
        // Creation of an array of ParentClass objects
        ParentClass[] functionality_Array = new ParentClass[500];
        // Display memory usage before creating the objects
        System.out.println("Memory Usage Before Creating Objects:");
        new ClassMemory(0, "").display_Memory_Usage();
        // Creation objects and store them in the array
        for (int i = 0; i < object_Array.length; i++) {
            object_Array[i] = new ClassMemory(i, "Object" + i);
        }
        // Creation ParentClass objects and store them in the array
        for (int i = 0; i < functionality_Array.length; i++) {
            functionality_Array[i] = new ParentClass("Functionality" + i);
        }
        // Printing memory usage and allocation after creating the objects
        System.out.println("Memory Usage After Creating Objects:");
        object_Array[0].display_Memory_Usage();
        for (ClassMemory obj : object_Array) {
            obj.additional_Task();
        }
        // Printing memory usage after performing additional work
        System.out.println("Memory Usage After Performing Additional Work:");
        object_Array[0].display_Memory_Usage();
        for (int i = 0; i < object_Array.length / 2; i++) {
            object_Array[i] = null; 
        }
        // Triggering garbage collection manually using System.gc() method.
        System.gc();
        // Printing the memory usage after garbage collection
        System.out.println("Memory Usage After Garbage Collection:");
        object_Array[0].display_Memory_Usage();
    }
}

输出

Memory Usage Before Creating Objects:
Total Memory: 123731968 bytes
Free Memory: 121716432 bytes
Used Memory: 2015536 bytes
------------------------
Memory Usage After Creating Objects:
Total Memory: 123731968 bytes
Free Memory: 121150408 bytes
Used Memory: 2581560 bytes
------------------------
Memory Usage After Performing Additional Work:
Total Memory: 123731968 bytes
Free Memory: 118347160 bytes
Used Memory: 5384808 bytes

让我们来看另一个相同的例子。

文件名: ClassMemoryDemo.java

class ChildClass {
    private double value;
    public ChildClass(double value) {
        this.value = value;
    }
    public void performTask() {
        for (int i = 0; i < 50; i++) {
            this.value *= i;
        }
    }
}
public class ClassMemoryDemo {
    private long longValue;
    private String stringValue;
    private ChildClass functionality;
    public ClassMemoryDemo(long longValue, String stringValue) {
        this.longValue = longValue;
        this.stringValue = stringValue;
        this.functionality = new ChildClass(3.14);
    }
    public void displayMemoryUsage() {
        long totalMemory = Runtime.getRuntime().totalMemory();
        long freeMemory = Runtime.getRuntime().freeMemory();
        long usedMemory = totalMemory - freeMemory;
        System.out.println("Total Memory Allocated: " + totalMemory + " bytes");
        System.out.println("Free Memory Allocated: " + freeMemory + " bytes");
        System.out.println("Used Memory Allocated: " + usedMemory + " bytes");
        System.out.println("------------------------");
    }
    public void additionalTask() {
        for (int i = 0; i < 50; i++) {
            this.longValue += i;
            this.stringValue += "Y";
        }
        this.functionality.performTask();
    }
    public static void main(String[] args) {
        // Creation of an array of objects
        ClassMemoryDemo[] objectArray = new ClassMemoryDemo[1000];
        // Creation of an array of ChildClass objects
        ChildClass[] functionalityArray = new ChildClass[500];
        // Display memory usage before creating the objects
        System.out.println("Memory Usage Before Creating Objects:");
        new ClassMemoryDemo(0, "").displayMemoryUsage();
        // Creating objects and storing them in the array
        for (int i = 0; i < objectArray.length; i++) {
            objectArray[i] = new ClassMemoryDemo(i, "Object" + i);
        }
        // Creating ChildClass objects and storing them in the array
        for (int i = 0; i < functionalityArray.length; i++) {
            functionalityArray[i] = new ChildClass(2.71);
        }
        // Printing memory usage and allocation after creating the objects
        System.out.println("Memory Usage After Creating Objects:");
        objectArray[0].displayMemoryUsage();
        for (ClassMemoryDemo obj : objectArray) {
            obj.additionalTask();
        }
        // Printing memory usage after performing additional work
        System.out.println("Memory Usage After Performing Additional Work:");
        objectArray[0].displayMemoryUsage();
        for (int i = 0; i < objectArray.length / 2; i++) {
            objectArray[i] = null;
        }
        // Triggering garbage collection manually using System.gc() method.
        System.gc();
        // Printing the memory usage after garbage collection
        System.out.println("Memory Usage After Garbage Collection:");
        objectArray[0].displayMemoryUsage();
    }
}

输出

Memory Usage Before Creating Objects:
Total Memory Allocated: 123731968 bytes
Free Memory Allocated: 121716432 bytes
Used Memory Allocated: 2015536 bytes
------------------------
Memory Usage After Creating Objects:
Total Memory Allocated: 123731968 bytes
Free Memory Allocated: 121150400 bytes
Used Memory Allocated: 2581568 bytes
------------------------
Memory Usage After Performing Additional Work:
Total Memory Allocated: 123731968 bytes
Free Memory Allocated: 117522224 bytes
Used Memory Allocated: 6209744 bytes
------------------------

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