JVM_内存_堆

1.堆_Heap_介绍

通过 new 关键字，创建对象都会使用堆内存

2.特点

1.它是线程共享的，堆中对象都需要考虑线程安全的问题
2.有垃圾回收机制【堆中没有被引用的对象，会被垃圾回收】

3.堆内存溢出

异常：java.lang.OutOfMemoryError: Java heap space
代码：
    public static void main(String[] args) {
        int i = 0;
        try {
            List<String> list = new ArrayList<>();
            //在catch之前 list 都有效，所以在catch之前不会被垃圾回收
            String a = "hello";
            while (true) {
                // hello, hellohello, hellohellohellohello ...
                list.add(a); 
                // hellohellohellohello
                a = a + a;  
                i++;
            }
        } catch (Throwable e) {
            e.printStackTrace();
            System.out.println(i);
        }
    }
参数：
    -Xmx8m  8m堆内存

4.堆内存诊断

//代码
public static void main(String[] args) throws InterruptedException {
        System.out.println("1...");
        Thread.sleep(30000);
        byte[] array = new byte[1024 * 1024 * 10]; // new占用了堆空间 10 Mb
        System.out.println("2...");
        Thread.sleep(20000);
        array = null;
        System.gc();
        System.out.println("3...");
        Thread.sleep(1000000L);
}
1. jps 工具
    查看当前系统中有哪些 java 进程

    10656 RemoteMavenServer36
    22112 Jps
    26020
    15896 Launcher
    21160 Demo1_4   //案例的进程id
============================================

2. jmap 工具 
    查看堆内存占用情况【内存快照】：jmap - heap 进程id
    eg：jmap -heap 21160

    //程序运行到2...的内存快照如下：
    Attaching to process ID 21160, please wait...
    Debugger attached successfully.
    Server compiler detected.
    JVM version is 25.172-b11

    using thread-local object allocation.
    Parallel GC with 13 thread(s)

    Heap Configuration://堆内存的配置
    MinHeapFreeRatio         = 0
    MaxHeapFreeRatio         = 100
    MaxHeapSize              = 4271898624 (4074.0MB)//最大堆内存
    NewSize                  = 89128960 (85.0MB)
    MaxNewSize               = 1423966208 (1358.0MB)//最大新生代内存
    OldSize                  = 179306496 (171.0MB)//最大老年代内存
    NewRatio                 = 2
    SurvivorRatio            = 8
    MetaspaceSize            = 21807104 (20.796875MB)
    CompressedClassSpaceSize = 1073741824 (1024.0MB)
    MaxMetaspaceSize         = 17592186044415 MB
    G1HeapRegionSize         = 0 (0.0MB)

    Heap Usage://堆内存的占用
    PS Young Generation
    Eden Space://新创建的对象都会占用伊甸园
    capacity = 67108864 (64.0MB)//总容量
    used     = 19886304 (18.965057373046875MB)//占用
    free     = 47222560 (45.034942626953125MB)
    29.632902145385742% used
    From Space:
    capacity = 11010048 (10.5MB)
    used     = 0 (0.0MB)
    free     = 11010048 (10.5MB)
    0.0% used
    To Space:
    capacity = 11010048 (10.5MB)
    used     = 0 (0.0MB)
    free     = 11010048 (10.5MB)
    0.0% used
    PS Old Generation
    capacity = 179306496 (171.0MB)
    used     = 0 (0.0MB)
    free     = 179306496 (171.0MB)
    0.0% used

    3168 interned Strings occupying 259800 bytes.
============================================

3. jconsole 工具
    图形界面的，多功能的监测工具，可以连续监测

5.现象_垃圾回收后，内存占用仍然很高

jvisualvm 工具
：点击 堆 dump 【堆转储】 查询，查找前20个最大的对象

public class Demo1_13 {

public static void main(String[] args) throws InterruptedException {
    List<Student> students = new ArrayList<>();
        for (int i = 0; i < 200; i++) {
            students.add(new Student());
            //Student student = new Student();
        }
        Thread.sleep(1000000000L);
    }
}
class Student {
    private byte[] big = new byte[1024*1024];
}