Java应用程序中每个类的成本 – 较少的大类或几个较小的类

For every new Class added to a Java application,what is the cost in terms of memory?

通常没关系.一般来说,只有一小部分(比如说5％)的整体内存使用被代码以各种形式占用.因此,即使您确实将代码大小减少了一半,整体内存使用量也只会略有下降.

相比之下,过长的源文件会使代码库难以导航,而较大的范围使得更难以全面了解类的功能以及某个更改是否安全.因此,长源文件会使修改代码变得更加昂贵且容易出错.

• On first use Class is loaded into memory (Memory used is roughly the size of the class file)

正确.

• If JIT is used,some additional machine-friendly binary version is created by the JIT compiler which uses a little more memory
• If Hotspot is used only some of the frequently used classes are optimised with machine-friendly versions (to balance memory and speed)

Hotspot是一个JIT,所以你在这里重复一遍.但是,是的,JIT确实增加了代码大小(但提高了速度).

Once a class is loaded,creating additional instances has negligible overhead (about 50-100 bytes?) (assuming no instance variables)

这是特定于JVM的.在Oracle Hotspot JVM上,每个对象的内存开销大约为8个字节,如以下程序所示：

public class Test {
    public static void main(String[] args) {
        Object[] array = new Object[10_000_000];
        Runtime rt = Runtime.getRuntime();
        long usedBefore = rt.totalMemory() - rt.freeMemory();
        for (int i = 0; i < array.length; i++ ) {
            array[i] = new Object();
        }
        long usedAfter = rt.totalMemory() - rt.freeMemory();
        System.out.println(usedBefore);
        System.out.println(usedAfter);
        System.out.println((double)(usedAfter - usedBefore) / array.length);
    }
}

• Once a class is loaded the class itself is never garbage collected

虽然它不是Java语言规范的强制要求,但我使用的每个JVM都会在ClassLoader无法访问时释放一个类(授予后,引导类加载器将始终保持可访问状态,但自定义ClassLoader可能无法访问).