Github操作

Posted on 2020-08-01 Valine:
Symbols count in article: 32 Reading time ≈ 1 mins.

in

stars/forks

awesome

#L

T

搜索某个区域的大佬

Linux常用命令

Posted on 2020-07-30 Edited on 2020-08-01 Valine:
Symbols count in article: 78 Reading time ≈ 1 mins.

top

load average 一分钟，五分钟，十五分钟

相加/3*100%大于60%说明负载重

uptime是top的精简命令

vmstat

free

iostat

优化

垃圾回收器

Posted on 2020-07-28 Edited on 2020-07-30 Valine:
Symbols count in article: 163 Reading time ≈ 1 mins.

四种常用的垃圾收集器

四大垃圾回收算法

引用计数

复制拷贝

标记清楚

标记整理

java -XX:+PrintCommandLineFlags -version

查看默认的垃圾回收器

java回收类型

串行收集器

并行收集器

ParNew

Parallel Scavenge

Parallel Old

CMS

Serial Old

G1

在springboot的具体调优

JavaOOM

Posted on 2020-07-27 Edited on 2020-07-28 In java Valine:
Symbols count in article: 1.8k Reading time ≈ 2 mins.

StackOverFlowError

public class StackOverFlowErrorDemo {

    public static void main(String[] args) {
        dfs();//Exception in thread "main" java.lang.StackOverflowError
    }

    public static void dfs(){
        dfs();
    }
}

最常见在递归中无返回条件一直调用。

JavaHeapSpace

public class JavaHeapSpaceDemo {

    public static void main(String[] args) {
        String str="wqoierihfsdkfsdf";

        while (true){
            str+=new Random().nextInt(108432432)+new Random().nextInt(238923293);
            str.intern();
        }
    }
}

GC Overhead Limited Exceeded

/**-Xms10m -Xmx10m -XX:+PrintGCDetails -XX:MaxDirectMemorySize=3m
 * @author xixing
 * @version 1.0
 * @date 2020/7/27 15:33
 */
public class GCOverheadDemo {

    public static void main(String[] args) {
        int i=0;

        List<String> list=new ArrayList<>();

        try {
            while (true){
                list.add(String.valueOf(i++).intern());
            }
        }catch (Throwable e){

            System.out.println("********************");
            System.out.println(i);
            e.printStackTrace();

        }
    }
}

DirectBufferMemory

/**
 * @author xixing
 * @version 1.0
 * @date 2020/7/27 15:49
 */
public class DirectBufferMemoryDemo {
    public static void main(String[] args) {


        System.out.println("配置的本地内存maxDirectMemory:"+sun.misc.VM.maxDirectMemory()/(double)1024/1024+"MB");
        Buffer bb= ByteBuffer.allocateDirect(10*1024*1024);
    }
}

Unable to create new native thread

/**
 * @author xixing
 * @version 1.0
 * @date 2020/7/27 16:08
 */
public class UnableCreateNewThreadDemo {
    public static void main(String[] args) {

        for (int i = 0; ; i++) {
            System.out.println("****************"+i);
            new Thread(()->{
                try {
                    Thread.sleep(Integer.MAX_VALUE);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            },String.valueOf(i)).start();
        }

    }
}

调整最大线程数

ulimit -u 查看可以创建的最大线程

vim /etc/security/limits.d/90-nproc.conf

Metaspace

Java引用

Posted on 2020-07-26 In java Valine:
Symbols count in article: 3.5k Reading time ≈ 3 mins.

强引用

public static void main(String[] args) {
    Object obj1=new Object();
    Object obj2=obj1;
    obj1=null;
    System.gc();
    System.out.println(obj2);
}

软引用

public class SoftReferenceDemo {

    /**
     * 内存够用就不回收，内存不够就回收
     */
    public static void softRefMemoryEnough(){
        Object o1=new Object();
        SoftReference<Object> softReference=new SoftReference<Object>(o1);
        System.out.println(o1);
        System.out.println(softReference.get());
        o1=null;
        System.gc();
        System.out.println(o1);
        System.out.println(softReference.get());

    }

    /**
     * 故意产生oom，new一个大内存数组，设置堆内存大小小一点
     * -Xms5m -Xmx5m
     */
    public static void softRefMemoryNotEnough(){
        Object o1=new Object();
        SoftReference<Object> softReference=new SoftReference<Object>(o1);
        System.out.println(o1);
        System.out.println(softReference.get());
        o1=null;

        try {
            byte[] bytes=new byte[30*1024*1024];
        }catch (Exception e) {
            e.printStackTrace();
        }finally
         {
            System.out.println(o1);
            System.out.println(softReference.get());
        }

    }

    public static void main(String[] args) {
        //softRefMemoryEnough();
        softRefMemoryNotEnough();
    }
}

弱引用

public class WeekReferenceDemo {

    public static void main(String[] args) {
        Object o1=new Object();
        WeakReference<Object> weakReference=new WeakReference<>(o1);
        System.out.println(o1);
        System.out.println(weakReference.get());
        System.out.println("==================");
        o1=null;
        System.gc();
        System.out.println(o1);
        System.out.println(weakReference.get());
    }
}

软引用和弱引用英语场景

WeakHashMap

public class WeakHashMapDemo {

    public static void main(String[] args) {
        myHashMap();
        System.out.println("==============================");
        myWeakHashMap();
    }



    private static void myHashMap() {
        Map<Integer,String> hashMap=new HashMap<>();
        Integer a=new Integer(1);
        hashMap.put(a,"HashMap");
        System.out.println(hashMap);

        a=null;
        System.gc();
        System.out.println(hashMap);
        System.out.println(hashMap);

    }

    private static void myWeakHashMap() {
        Map<Integer,String> hashMap=new WeakHashMap<>();
        Integer a=new Integer(1);
        hashMap.put(a,"HashMap");
        System.out.println(hashMap);

        a=null;
        System.out.println(hashMap);
        System.gc();
        System.out.println(hashMap);

    }
}

虚引用

public class PhantomReferenceDemo {

    public static void main(String[] args) throws InterruptedException {
        Object o1=new Object();
        ReferenceQueue<Object> referenceQueue=new ReferenceQueue<>();
        PhantomReference<Object> phantomReference=new PhantomReference<>(o1,referenceQueue);

        System.out.println(o1);
        System.out.println(phantomReference.get());
        System.out.println(referenceQueue.poll());

        System.out.println("====================");
        o1=null;
        System.gc();
        Thread.sleep(500);

        System.out.println(o1);
        System.out.println(phantomReference.get());
        System.out.println(referenceQueue.poll());
    }
}

引用队列

public class ReferenceQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        Object o1=new Object();
        ReferenceQueue<Object> referenceQueue=new ReferenceQueue<>();
        WeakReference<Object> weakReference=new WeakReference<>(o1,referenceQueue);

        System.out.println(o1);
        System.out.println(weakReference.get());
        System.out.println(referenceQueue.poll());

        System.out.println("====================");
        o1=null;
        System.gc();
        Thread.sleep(500);

        System.out.println(o1);
        System.out.println(weakReference.get());
        System.out.println(referenceQueue.poll());

    }
}

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