JVM 内存分配与回收学习（1）

随笔。记录各类型在堆内存中占用内存空间大小的理解：

引用类型在堆中占用4字节，

byte，boolean基本类型在堆中占用1字节，

char，short基本类型在堆中占用2字节，

int，float基本类型在堆中占用4字节，

long，double基本类型在堆中占用8字节。

用代码验证下，以下代码的vm参数为

-Xmx20m -Xms20m -XX:+HeapDumpOnOutOfMemoryError

-XX:HeapDumpPath=d:\dump -XX:+PrintGCDetails -Xmn10M

-Xmx20m：JVM heap 最大分配堆内存为20m

-Xms20m：JVM heap 初始分配堆内存为20m

-XX:+HeapDumpOnOutOfMemoryError 表示内存溢出时，dump当前的堆转储快照

-XX:HeapDumpPath=d:\dump 堆转储快照的存放路径

-XX:+PrintGCDetails JVM 发现GC时打印GC日志

-Xmn10M:年轻代堆的大小为10m

1、测试new String[1024*1024]占用堆内存空间的大小

public class TestOutOfMemory {
     
     public static void main(String[] args) {
          int v = 0;
          while(v<1) {
              String[] a = new String[1024*1024];
              System.out.println(v++);
          }
     }
}

打印heap内存发现

Heap
 def new generation   total 9216K, used 5136K [0x33400000, 0x33e00000, 0x33e00000)
  eden space 8192K,  62% used [0x33400000, 0x33904170, 0x33c00000)
  from space 1024K,   0% used [0x33c00000, 0x33c00000, 0x33d00000)
  to   space 1024K,   0% used [0x33d00000, 0x33d00000, 0x33e00000)
 tenured generation   total 10240K, used 0K [0x33e00000, 0x34800000, 0x34800000)
   the space 10240K,   0% used [0x33e00000, 0x33e00000, 0x33e00200, 0x34800000)
 compacting perm gen  total 12288K, used 183K [0x34800000, 0x35400000, 0x38800000)
   the space 12288K,   1% used [0x34800000, 0x3482dc68, 0x3482de00, 0x35400000)
    ro space 10240K,  44% used [0x38800000, 0x38c7c1d8, 0x38c7c200, 0x39200000)
    rw space 12288K,  52% used [0x39200000, 0x398440c0, 0x39844200, 0x39e00000)

“def new generation total 9216K, used 5136K [0x33400000, 0x33e00000, 0x33e00000)”从此处知，

占用的内容空间是4M。说明new String()在堆中占用的空间是4字节。

2、测试new char[1024*1024]占用堆内存空间的大小

public class TestOutOfMemory {
     
     public static void main(String[] args) {
          int v = 0;
          while(v<1) {
              char[] a = new char[1024*1024];
              System.out.println(v++);
          }
     }
}

打印heap内存发现，

“def new generation total 9216K, used 3088K [0x33400000, 0x33e00000, 0x33e00000)”

占用的内存空间为2M，说明new char()在堆中占用的空间是2字节。

0
Heap
 def new generation   total 9216K, used 3088K [0x33400000, 0x33e00000, 0x33e00000)
  eden space 8192K,  37% used [0x33400000, 0x33704098, 0x33c00000)
  from space 1024K,   0% used [0x33c00000, 0x33c00000, 0x33d00000)
  to   space 1024K,   0% used [0x33d00000, 0x33d00000, 0x33e00000)
 tenured generation   total 10240K, used 0K [0x33e00000, 0x34800000, 0x34800000)
   the space 10240K,   0% used [0x33e00000, 0x33e00000, 0x33e00200, 0x34800000)
 compacting perm gen  total 12288K, used 183K [0x34800000, 0x35400000, 0x38800000)
   the space 12288K,   1% used [0x34800000, 0x3482dc60, 0x3482de00, 0x35400000)
    ro space 10240K,  44% used [0x38800000, 0x38c7c1d8, 0x38c7c200, 0x39200000)
    rw space 12288K,  52% used [0x39200000, 0x398440c0, 0x39844200, 0x39e00000)

JVM 内存分配与回收学习（1）

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