堆 vs 堆栈 vs 彼尔姆空间
2022-09-01 02:19:46
- Java内存空间(Perm Space,Space Stack,Heap Space)之间有什么区别?
- JVM 何时使用一个或另一个?
- 如果我使用Scala/Groovy/等,有区别吗?
只是
例如:
Student std = new Student();
执行完行后,上面的内存状态会是这样的。
原谅我为这样一个老问题添加了一个答案 - 目前的答案很好,但由于静态代码和Java 8更新而错过了几个边缘情况。
概述
示例代码
public class SimpleVal { //The Class (loaded by a classloader) is in the PermGen
private static final int MAGIC_CONSTANT = 42; //Static fields are stored in PermGen
private static final SimpleVal INSTANCE = new SimpleVal(1); //Static field objects are created in the heap normally, with the reference in the PermGen ('class statics' moved to the heap from Java 7+)
private static SimpleVal previousInstance; //Mutable static fields also have their reference in PermGen so they can easily cause memory leaks
private int value; //Member variables will be part of the heap
public SimpleVal(int realValue) {
value = realValue;
...
}
public static int subtract(SimpleVal val1, SimpleVal val2) {
....
}
public int add(SimpleVal other) { //Only one copy of any method (static or not) exists - in PermGen
int sum = value + other.value; //Local values in methods are placed in the Stack memory
return sum;
}
}
public static void main(String[] args) {
SimpleVal val1 = null;
SimpleVal val2 = new SimpleVal(3); //Both of these variables (references) are stored in the Stack
val1 = new SimpleVal(14); //The actual objects we create and add to the variables are placed in the Heap (app global memory, initially in the Young Gen space and later moved to old generation, unless they are very large they can immediately go old gen)
int prim = val1.add(val2); //primitive value is stored directly in the Stack memory
Integer boxed = new Integer(prim); //but the boxed object will be in the heap (with a reference (variable) in the Stack)
String message = "The output is: "; //In Java 7+ the string is created in the heap, in 6 and below it is created in the PermGen
System.out.println(message + prim);
}
Java 8 注意:PermGen空间被所谓的Metaspace所取代。这仍然具有相同的功能,但可以自动调整大小 - 默认情况下,Metaspace自动将其在本机内存中的大小增加到最大值(在JVM参数中指定),但PermGen始终具有与堆内存相邻的固定最大大小。
安卓笔记:从Android 4.0(在实践中从3.0开始)Android应该遵守所描述的内存契约 - 但在旧版本上,实现被破坏了。Android-Davlik中的“Stack”内存实际上是基于寄存器的(指令大小和计数在两者之间有所不同,但对于开发人员来说,功能保持不变)。
最后,欲了解更多信息,我在StackOverflow上看到的关于这个主题的最佳答案在这里