简单
例如:
Student std = new Student();
执行上面这行之后,内存状态会是这样的。
请原谅我为这样一个老问题添加答案 - 当前的答案很好,但由于静态代码和 Java 8 更新而错过了一些边缘情况。
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 空间被所谓的元空间取代。它的功能仍然相同,但可以自动调整大小 - 默认情况下,元空间会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。
Android 注意:从 Android 4.0(实际上从 3.0 开始)Android 应该遵守所描述的内存合约 - 但在旧版本上,实现已被破坏。 Android-Dalvik 中的“堆栈”内存实际上是基于寄存器的(两者之间的指令大小和计数有所不同,但对于开发人员而言,功能保持不变)。
最后,要了解更多信息,我在 StackOverflow 上见过的关于此主题的最佳答案是这里