The j360-tools from almouszhang

j360-tools Java知识点持续更新

开源爱好者, 善于Java体系微服务架构和运维优化, 学而思, 思而学, 分享一下工作中使用的技术点, 会持续更新

书单篇

我的书单: 2019夏至篇

基础篇

在String中使用KMP算法计算部分操作
JMH验证
线程池如何用好/shutdown pool时的规则, shutdown() && shutdownnow(), kill和钩子引用
二叉堆实现有序队列
解读ThreadLocal
Java中的排序场景,Collections.sort, TreeMap
线程生命周期各状态在jstack中的解读
String.intern, Long, Integer等对象池在jvm中使用
finalize,phantomReference使用
验证hash、一致性hash的分布(murmurhash)
算法:sort/search/rate limit/sliding window
索引,B-tree、invert index
对象头、指针、锁、类
SecureRandom seed相关
异常
异步 & Future, 并发异步读提升接口性能
ForkJoin工作原理
nio对照阻塞io写法,Selector工作原理

提高篇

事务,分布式事务,Innodb实现
- 事务,分布式事务,Innodb实现,分布式事务案例分析
- 分布式事务Alibaba Fescar实现
一致性hash分片、扩容与缩服
CAP
redis & cluster
JVM VMFlags, Hotspot/JPDA/JVMTI相关参考:
- https://www.ibm.com/developerworks/cn/java/j-lo-jpda3/index.html
- http://calvin1978.blogcn.com/articles/jvmoption-7.html
Netty/Selector在工程中的高效使用 Netty案例
Log4j2的高效使用
HikariCP 解读、译文
Web服务会话管理实现

运维篇

coredump segmentfault https://www.cnblogs.com/lidabo/p/5014710.html
句柄、TCP/内核相关调整 ulimit -n
netstat 状态解读
crontab 简单搞定定时器,备份日志、mysql、任务执行调度
Mysql连接池
Java服务化shell
日志利器sed、awk
Nginx常用配置及优化手段
redis: master/slave,cluster

常用中间件工具篇

elasticsearch
canal
flume/fluentd
zipkin/brave/Skywalking
Azkaban
Apollo
Zabbix/Kibana/Grafana
ActiveMQ
RocketMQ v4.3.0
Dubbo
Sharding-Sphere
Saga
Kafka
Seata

容器篇

docker/docker compose
k8s
istio

书单

https://github.com/xuminwlt/j360-book-list

业务篇

Timeline
消息、推送系统
用户
支付
计数器

问题排查篇

慢接口
GC
Mysql死锁

开发小技巧篇

分布式锁
Master-Slave
限流
热点数据

基础篇

1. KMP算法+JMH验证

# Measurement: 2 iterations, 1 s each
# Timeout: 10 min per iteration
# Threads: 1 thread, will synchronize iterations
# Benchmark mode: Throughput, ops/time
# Benchmark: me.j360.tools.ToolsBenchmark.indexOfKMPLongText

...
Result "me.j360.tools.ToolsBenchmark.indexOfStringLongText":
  1171034.703 ops/ms


# Run complete. Total time: 00:00:18

Benchmark                              Mode  Cnt        Score   Error   Units
ToolsBenchmark.indexOfKMP             thrpt    2    22165.252          ops/ms
ToolsBenchmark.indexOfKMPLongText     thrpt    2     1343.742          ops/ms
ToolsBenchmark.indexOfString          thrpt    2  2957587.750          ops/ms
ToolsBenchmark.indexOfStringLongText  thrpt    2  1171034.703          ops/ms

3. 理解线程池,用好线程池

从使用到深入了解自定义线程池ThreadPoolExecutor,先了解最完整的构造方法参数,一共7个参数

/**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters.
     *
     * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the
     *        pool
     * @param keepAliveTime when the number of threads is greater than
     *        the core, this is the maximum time that excess idle threads
     *        will wait for new tasks before terminating.
     * @param unit the time unit for the {@code keepAliveTime} argument
     * @param workQueue the queue to use for holding tasks before they are
     *        executed.  This queue will hold only the {@code Runnable}
     *        tasks submitted by the {@code execute} method.
     * @param threadFactory the factory to use when the executor
     *        creates a new thread
     * @param handler the handler to use when execution is blocked
     *        because the thread bounds and queue capacities are reached
     * @throws IllegalArgumentException if one of the following holds:<br>
     *         {@code corePoolSize < 0}<br>
     *         {@code keepAliveTime < 0}<br>
     *         {@code maximumPoolSize <= 0}<br>
     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue}
     *         or {@code threadFactory} or {@code handler} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

参数列表核心线程数:默认参照cpu的数量最大线程数:线程池所能容纳的最大线程数空闲存活时间:非核心线程的空闲存活时间时间单位队列实例:任务队列,其中常用的有三种队列，SynchronousQueue,LinkedBlockingDeque,ArrayBlockingQueue 线程工厂:尽量自定义线程工厂,默认DefaultThreadFactory 拒绝策略:默认丢弃,选择jdk自带的四选一策略。
内部属性 ctl:记录每个线程的状态字段,使用29+3位的Integer类型记录,当数量更大时,未来会考虑使用AtomicLong类型
参数大小特别说明
1. ** 当线程池小于corePoolSize时，新提交任务将创建一个新线程执行任务，即使此时线程池中存在空闲线程。 **
2. 当线程池达到corePoolSize时，新提交任务将被放入workQueue中，等待线程池中任务调度执行
3. 当workQueue已满，且maximumPoolSize>corePoolSize时，新提交任务会创建新线程执行任务
4. 当提交任务数超过maximumPoolSize时，新提交任务由RejectedExecutionHandler处理
5. 当线程池中超过corePoolSize线程，空闲时间达到keepAliveTime时，关闭空闲线程
6. 当设置allowCoreThreadTimeOut(true)时，线程池中corePoolSize线程空闲时间达到keepAliveTime也将关闭
线程池生命周期

创建:构造方法,核心线程此时还是空
运行:提交任务,根据线程数量等情况创建线程或拒绝
停止:shutdown()->拒绝新提交线程->阻塞一直等待线程池中线程+队列中任务全部完成->销毁线程池, shutdownnow()->尝试停止正在运行的线程,丢弃当前等待的所有线程和队列任务,直接销毁线程池,当尝试停止的线程忽略interrupt操作时,可能永远无法真正停止

gc相关

/**
     * Invokes {@code shutdown} when this executor is no longer
     * referenced and it has no threads.
     */
    protected void finalize() {
        shutdown();
    }

这里覆写了finalize方法,在垃圾回收时还会再次进行shutdown操作,具体参见《9. Finalize》

延伸:ForkJoinPool 1.7+

ForkJoinPool使用了 @sun.misc.Contended 注解,表示此处使用了CPU缓存管理的行填充方式 ForkJoinPool使用work-stealing工作方式,由事件驱动,并支持1.8+lambda函数表达式在JDK异步执行的某些多态api中,如未使用线程池参数,默认使用ForkJoinPool.commonPool作为默认的线程池,该默认线程池使用jvm参数配置方式进行部分自定义操作

完整的构造方法

/**
     * Creates a {@code ForkJoinPool} with the given parameters.
     *
     * @param parallelism the parallelism level. For default value,
     * use {@link java.lang.Runtime#availableProcessors}.
     * @param factory the factory for creating new threads. For default value,
     * use {@link #defaultForkJoinWorkerThreadFactory}.
     * @param handler the handler for internal worker threads that
     * terminate due to unrecoverable errors encountered while executing
     * tasks. For default value, use {@code null}.
     * @param asyncMode if true,
     * establishes local first-in-first-out scheduling mode for forked
     * tasks that are never joined. This mode may be more appropriate
     * than default locally stack-based mode in applications in which
     * worker threads only process event-style asynchronous tasks.
     * For default value, use {@code false}.
     * @throws IllegalArgumentException if parallelism less than or
     *         equal to zero, or greater than implementation limit
     * @throws NullPointerException if the factory is null
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public ForkJoinPool(int parallelism,
                        ForkJoinWorkerThreadFactory factory,
                        UncaughtExceptionHandler handler,
                        boolean asyncMode) {
        this(checkParallelism(parallelism),
             checkFactory(factory),
             handler,
             asyncMode ? FIFO_QUEUE : LIFO_QUEUE,
             "ForkJoinPool-" + nextPoolId() + "-worker-");
        checkPermission();
    }

4. 二叉堆

二叉堆是有序队列场景常用的数据结构,Java的有序队列使用的就是二叉堆PriorityQueue

SortedList使用常规的归并排序进行sort操作(mergeSort),同时也是Collections.sort操作的排序方式
Array.sort使用TimSort进行排序,除非用户指定使用mergeSort
TimSort,来源于Python,使用优化过的二分插入排序

5. ThreadLocal

ThreadLocal是本地线程对象,在几乎所有的中间件框架中都有定义,通常用于存取当前线程的一些本地状态,比如slf4j中的MDC,Trace中的Tracer,各种容器中的Context上下文对象 ThreadLocal是如何做到的?

ThreadLocal只有一个构造方法new ThreadLocal(),泛型用于指定该ThreadLocal存储的对象类型,常用方法get(),set() 面试的大部分都知道但是细节都说错了,注意key并不是当前线程,getMap(t)仅仅是从当前线程中拿到ThreadLocalMap ThreadLocal使用一个Map对象存储所有当前线程的所有ThreadLocal对象,其中key=this->当前ThreadLocal,value=存储的对象当前线程对象中绑定了一个ThreadLocalMap对象称为threadLocals, 初始化时默认为空,当且仅当该线程为子线程需要继承父线程的threadLocal状态时会初始化inheritableThreadLocals ThreadLocal在初始化时会去给当前Thread对象绑定上threadLocals,从而在任何地方都可以通过

Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);

拿到ThreadLocalMap,从而可以拿到ThreadLocal对象

public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

ThreadLocal对象既然可以贯穿整个线程生命周期,那会不会存在gc问题,一般情况下不需要特别考虑手动回收,但是极端情况下会存在gc问题 Thread对象是gc的root根节点,gc时根据可达性分析来判断是否对对象进行回收当ThreadLocal中存储的对象未当前Thread中的对象时,发生gc时会回收掉所有相关对象,如果当前线程是线程池中的核心时,该线程并不会被销毁。

ThreadLocalMap使用ThreadLocal的弱引用作为key，如果一个ThreadLocal没有外部强引用来引用它，那么系统 GC 的时候，这个ThreadLocal势必会被回收，这样一来，ThreadLocalMap中就会出现key为null的Entry，就没有办法访问这些key为null的Entry的value，如果当前线程再迟迟不结束的话，这些key为null的Entry的value就会一直存在一条强引用链：Thread Ref -> Thread -> ThreaLocalMap -> Entry -> value永远无法回收，造成内存泄漏。其实，ThreadLocalMap的设计中已经考虑到这种情况，也加上了一些防护措施：在ThreadLocal的get(),set(),remove()的时候都会清除线程ThreadLocalMap里所有key为null的value。

但是这些被动的预防措施并不能保证不会内存泄漏：

使用static的ThreadLocal，延长了ThreadLocal的生命周期，可能导致的内存泄漏（参考ThreadLocal 内存泄露的实例分析）。
分配使用了ThreadLocal又不再调用get(),set(),remove()方法，那么就会导致内存泄漏。

6. 排序相关

Collections.sort:参考4 TreeMap/TreeSet中的Tree是红黑树的实现的排序方式,参考红黑树原理,红黑树在1.8+ HashMap/ConcurrentHashMap中的某些条件下链表会转化成红黑树,提升性能 LinkedHashMap使用的是顺序而不是排序,顺序方式为链表

7. jstack

1. synchronize锁下拿到锁的线程占据了cpu控制权,其他等待锁的block阻塞
1. 在1情况下的线程如果执行wait或者wait(time),则有条件出让锁和cpu资源,竞争2线程拿到锁,,1线程wait状态,等待2线程notify
1. 1线程情况下,如果线程sleep仅仅是线程sleep,进入wait状态,但是不出让cpu资源,等待sleep结束自我唤醒
1. wait、wait_time发生的情况有2中,1自己拿到锁,自己睡眠sleep,2自己拿到锁,自己wait
1. 其他线程执行完并notify时,wait线程收到通知进入竞争状态,生产者消费者场景是该问题排查的常用场景

min-xufpdeMacBook-Pro% jstack 73952
2018-07-05 14:41:24
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.101-b13 mixed mode):

"Attach Listener" #14 daemon prio=9 os_prio=31 tid=0x00007f838a061800 nid=0xd07 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"DestroyJavaVM" #13 prio=5 os_prio=31 tid=0x00007f83890a0800 nid=0x2603 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"pool-1-thread-3" #12 prio=5 os_prio=31 tid=0x00007f83890a0000 nid=0x3b03 waiting on condition [0x0000700003fb9000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
        at java.lang.Thread.sleep(Native Method)
        at me.j360.tools.ref.JstackCase$Task3.run(JstackCase.java:63)
        - locked <0x00000007956f70f0> (a java.lang.Object)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)

"pool-1-thread-2" #11 prio=5 os_prio=31 tid=0x00007f838a060800 nid=0x3e03 waiting for monitor entry [0x0000700003eb6000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x00000007956f70f0> (a java.lang.Object)
        at me.j360.tools.ref.JstackCase$Task.run(JstackCase.java:39)
        - locked <0x00000007956f70f0> (a java.lang.Object)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)

"pool-1-thread-1" #10 prio=5 os_prio=31 tid=0x00007f838a02b000 nid=0x3a03 waiting for monitor entry [0x0000700003db3000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x00000007956f70f0> (a java.lang.Object)
        at me.j360.tools.ref.JstackCase$Task.run(JstackCase.java:39)
        - locked <0x00000007956f70f0> (a java.lang.Object)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)

"Monitor Ctrl-Break" #9 daemon prio=5 os_prio=31 tid=0x00007f8389897000 nid=0x3903 runnable [0x0000700003cb0000]
   java.lang.Thread.State: RUNNABLE
        at java.net.PlainSocketImpl.socketAccept(Native Method)
        at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:409)
        at java.net.ServerSocket.implAccept(ServerSocket.java:545)
        at java.net.ServerSocket.accept(ServerSocket.java:513)
        at com.intellij.rt.execution.application.AppMain$1.run(AppMain.java:90)
        at java.lang.Thread.run(Thread.java:745)

"Service Thread" #8 daemon prio=9 os_prio=31 tid=0x00007f838a844000 nid=0x3803 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread2" #7 daemon prio=9 os_prio=31 tid=0x00007f838a058000 nid=0x3703 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" #6 daemon prio=9 os_prio=31 tid=0x00007f838a057000 nid=0x3503 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #5 daemon prio=9 os_prio=31 tid=0x00007f838a055800 nid=0x4603 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=31 tid=0x00007f838a055000 nid=0x3303 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=31 tid=0x00007f838901a000 nid=0x4f03 in Object.wait() [0x000070000359b000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x0000000795588ee0> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
        - locked <0x0000000795588ee0> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)

"Reference Handler" #2 daemon prio=10 os_prio=31 tid=0x00007f8389017800 nid=0x2d03 in Object.wait() [0x0000700003498000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x0000000795586b50> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(Object.java:502)
        at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
        - locked <0x0000000795586b50> (a java.lang.ref.Reference$Lock)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

"VM Thread" os_prio=31 tid=0x00007f838a040800 nid=0x5203 runnable 

"GC task thread#0 (ParallelGC)" os_prio=31 tid=0x00007f838981c000 nid=0x1d07 runnable 

"GC task thread#1 (ParallelGC)" os_prio=31 tid=0x00007f838a000800 nid=0x1e03 runnable 

"GC task thread#2 (ParallelGC)" os_prio=31 tid=0x00007f838a001000 nid=0x2b03 runnable 

"GC task thread#3 (ParallelGC)" os_prio=31 tid=0x00007f838a002000 nid=0x5303 runnable 

"VM Periodic Task Thread" os_prio=31 tid=0x00007f8389831000 nid=0x4203 waiting on condition 

JNI global references: 21

8. String池、自动装箱缓存池

String的池化是通过原生方法intern()来进行的,使用方式为new String().intern()来实现对象池的功能,该处是否应该翻译为常量池我认为是有争议的,常量池一般理解为存放在方法区,然而1.7+之后的String的池化操作是作为对象一样存放在堆中。 String.intern是一个原生方法,在底层的C++操作中,JVM为生成一个HashTable数据结构来作为String的缓存池,key为String的hashcode 既然该HashTable对象是存放在堆中,就会存在gc对该对象对垃圾回收的影响,从你假笨的文章中理解到

引用一下: JVM源码分析之String.intern()导致的YGC不断变长，其原因是YGC过程需要对StringTable做扫描，而String.intern()就是在StringTable中保存这个对象的引用，如果String.intern()添加越来越多不同的对象，那么StringTable就越大，扫描StringTable的时间就越长，从而导致YGC耗时越长；那么如何确定YGC耗时越来越长是StringTable变大引起的呢？

介绍一个参数-XX:+PrintStringTableStatistics，看名字就知道这个参数的作用了：打印出StringTable的统计信息；再详细一点描述：在JVM进程退出时，打印出StringTable的统计信息到标准日志输出目录中。

参考: https://www.jianshu.com/p/5524fce8b08f

同样在使用部分包装类时,也会存在相似的情况

String a = "a";
Long b = 1L;
Long c = Long.valueOf(1L);

源码可以看出,此时会默认生成一个LongCache的对象,同时会创建一个静态的Long的数组常量#1,并在类的初始化时给数组常量中每个对象赋值#2

private static class LongCache {
        private LongCache(){}

        //#1
        static final Long cache[] = new Long[-(-128) + 127 + 1];
        //#2
        static {
            for(int i = 0; i < cache.length; i++)
                cache[i] = new Long(i - 128);
        }
    }

1、Integer、Short、Character、Bytes等封装类也有类似的机制；
2、请关注JVM参数：AutoBoxCacheMax,仅对Integer有效

参考: https://blog.csdn.net/chengzhezhijian/article/details/9628251

9. Finalize

RunFinalize.java
PhantomReference: 在垃圾回收时收到一个系统通知

直接内存的回收

Cleaner是PhantomReference的子类，并通过自身的next和prev字段维护的一个双向链表。PhantomReference的作用在于跟踪垃圾回收过程，并不会对对象的垃圾回收过程造成任何的影响。

复写

vm -verbose:gc -Xloggc:gc_%p.log -XX:+PrintGC -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintHeapAtGC -XX:+PrintGCTimeStamps -XX:+PrintTenuringDistribution -XX:+PrintGCApplicationStoppedTime

只有在finalize中再次引用对象本身才会发生对象复活1次的行为,在执行System.gc时因为对象回收的不确定性,所以根据gc日子判断回收问题,并不能非常准确的描述出事件发生节点结论如下: 对象复活的行为必须是对象本身在垃圾回收时,在finalize引用一次同时再次执行垃圾回收时,该对象必被回收,这是回收时间点不确定根据gc日志的节点只能作为参考,不能作为实验判断依据

jdk7+ 终止方法中在try withresource使用场景,在finalize中实现非内存资源的释放, file/ inputstream/ socket等等
Finalizer类和FinalizerThread最终实现垃圾回收的工作,并因为其优先级的原因,无法做到准确的预测时间
虚拟引用比其他引用可以更安全的重写finalize导致的问题,作为垃圾回收清理时的通知机制的特殊需求参考: https://www.jianshu.com/p/9d2788fffd5f http://www.cnblogs.com/jqyp/archive/2010/11/27/1889414.html http://zhang-xzhi-xjtu.iteye.com/blog/413159

Java HotSpot(TM) 64-Bit Server VM (25.101-b13) for bsd-amd64 JRE (1.8.0_101-b13), built on Jun 22 2016 02:42:15 by "java_re" with gcc 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)
Memory: 4k page, physical 8388608k(104744k free)

/proc/meminfo:

CommandLine flags: -XX:InitialHeapSize=134217728 -XX:MaxHeapSize=2147483648 -XX:+PrintGC -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC -XX:+PrintTenuringDistribution -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC 
{Heap before GC invocations=1 (full 0):
 PSYoungGen      total 38400K, used 2662K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
  eden space 33280K, 8% used [0x0000000795580000,0x0000000795819a28,0x0000000797600000)
  from space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
  to   space 5120K, 0% used [0x0000000797600000,0x0000000797600000,0x0000000797b00000)
 ParOldGen       total 190464K, used 102400K [0x0000000740000000, 0x000000074ba00000, 0x0000000795580000)
  object space 190464K, 53% used [0x0000000740000000,0x0000000746400010,0x000000074ba00000)
 Metaspace       used 2998K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 331K, capacity 388K, committed 512K, reserved 1048576K
2018-07-05T13:28:43.683-0800: 0.382: [GC (System.gc()) 
Desired survivor size 5242880 bytes, new threshold 7 (max 15)
[PSYoungGen: 2662K->496K(38400K)] 105062K->102904K(228864K), 0.0023376 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap after GC invocations=1 (full 0):
 PSYoungGen      total 38400K, used 496K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
  eden space 33280K, 0% used [0x0000000795580000,0x0000000795580000,0x0000000797600000)
  from space 5120K, 9% used [0x0000000797600000,0x000000079767c010,0x0000000797b00000)
  to   space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
 ParOldGen       total 190464K, used 102408K [0x0000000740000000, 0x000000074ba00000, 0x0000000795580000)
  object space 190464K, 53% used [0x0000000740000000,0x0000000746402010,0x000000074ba00000)
 Metaspace       used 2998K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 331K, capacity 388K, committed 512K, reserved 1048576K
}
{Heap before GC invocations=2 (full 1):
 PSYoungGen      total 38400K, used 496K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
  eden space 33280K, 0% used [0x0000000795580000,0x0000000795580000,0x0000000797600000)
  from space 5120K, 9% used [0x0000000797600000,0x000000079767c010,0x0000000797b00000)
  to   space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
 ParOldGen       total 190464K, used 102408K [0x0000000740000000, 0x000000074ba00000, 0x0000000795580000)
  object space 190464K, 53% used [0x0000000740000000,0x0000000746402010,0x000000074ba00000)
 Metaspace       used 2998K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 331K, capacity 388K, committed 512K, reserved 1048576K
2018-07-05T13:28:43.685-0800: 0.384: [Full GC (System.gc()) [PSYoungGen: 496K->0K(38400K)] [ParOldGen: 102408K->102800K(190464K)] 102904K->102800K(228864K), [Metaspace: 2998K->2998K(1056768K)], 0.0080959 secs] [Times: user=0.01 sys=0.00, real=0.01 secs] 
Heap after GC invocations=2 (full 1):
 PSYoungGen      total 38400K, used 0K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
  eden space 33280K, 0% used [0x0000000795580000,0x0000000795580000,0x0000000797600000)
  from space 5120K, 0% used [0x0000000797600000,0x0000000797600000,0x0000000797b00000)
  to   space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
 ParOldGen       total 190464K, used 102800K [0x0000000740000000, 0x000000074ba00000, 0x0000000795580000)
  object space 190464K, 53% used [0x0000000740000000,0x0000000746464098,0x000000074ba00000)
 Metaspace       used 2998K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 331K, capacity 388K, committed 512K, reserved 1048576K
}

.... 

2018-07-05T13:28:44.714-0800: 1.412: Total time for which application threads were stopped: 0.0071747 seconds, Stopping threads took: 0.0000322 seconds
Heap
 PSYoungGen      total 38400K, used 665K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
  eden space 33280K, 2% used [0x0000000795580000,0x00000007956267d8,0x0000000797600000)
  from space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
  to   space 5120K, 0% used [0x0000000797600000,0x0000000797600000,0x0000000797b00000)
 ParOldGen       total 190464K, used 379K [0x0000000740000000, 0x000000074ba00000, 0x0000000795580000)
  object space 190464K, 0% used [0x0000000740000000,0x000000074005ef78,0x000000074ba00000)
 Metaspace       used 3006K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 332K, capacity 388K, committed 512K, reserved 1048576K

10. hash,一致性hash

hash在日常使用中有很多相关的场景和工具,比如MD5、SHA1/256/512等等,Java中常见的hash就是每个Object对象的hashcode,他们的作用只有一个就是对一个数据进行某种算法后得到一个确定的分散的值,俗称散列。常见的散列算法都能实现散列的均匀分布,当分布中的某些区间因为某些情况需要变更时,就存在容错性和扩展性方面的欠缺了,这些场景大量用在分布式的场景中,此时就需要使用一致性hash算法来完成。

常见的一致性hash实现之一MurmurHash能够很好的解释一致性hash的原理,一致性hash使用一些特殊的概念 - 哈希环 - 虚拟节点

参考: http://calvin1978.blogcn.com/articles/murmur.html

11. 算法:sort/search/sliding window/rate limit

排序算法:插入排序、希尔排序、选择排序、堆排序、冒泡排序、快速排序、归并排序、基数排序
查找算法:顺序查找、二分查找、插值查找、斐波那契查找、树表查找、分块查找、哈希查找

https://www.cnblogs.com/maybe2030/p/4715042.html http://www.cnblogs.com/maybe2030/p/4715035.html

滑动窗口: https://blog.csdn.net/wdscq1234/article/details/52444277 http://yunchow.iteye.com/blog/2277593

限流:

海量处理处理结构

分而治之/hash映射 + hash统计 + 堆/快速/归并排序
双层桶划分
Bloom filter/Bitmap
Trie树(字典树)/数据库/倒排索引
外排序
分布式处理之Hadoop/Mapreduce

12.索引:B-tree、invert index、GeoHash

TODO

13. 对象头、指针、锁、类

先从Synchronize关键字来看 synchronized 关键字是解决并发问题常用解决方案，有以下三种使用方式:

同步普通方法，锁的是当前对象。同步静态方法，锁的是当前 Class 对象。同步块，锁的是 () 中的对象。

参考: https://github.com/crossoverJie/JCSprout/blob/master/MD/Synchronize.md

14. SecureRandom seed相关

参考: http://calvin1978.blogcn.com/articles/securerandom.html

15. 异常

异常在Java中常见的关键字有: Throwable Throw Throws Error Exception 异常相关的内容:

异常类的设计
如何设计异常、抛出异常、捕获异常
如何用好异常

异常类的设计

理解异常类的源码和设计对异常的使用有很大的帮助,Throwable是异常的顶级类,封装了异常相关的数据结构和常用方法

业务统一的输出异常

使用静态异常,使用clone和消除异常trace,能够大幅度提高业务异常在系统中的性能。参考

long start = (System.currentTimeMillis());
        BizException bizException = BizException.bizException;
        System.out.println(System.currentTimeMillis()  - start);
        for (int i= 0;i < 1000; i++) {
            try {
                throw bizException.clone(DefaultErrorCode.SYS_ERROR);
            } catch (Exception e) {
                //System.out.println(e);
            }
        }
        System.out.println(System.currentTimeMillis()  - start);
        for (int i= 0;i < 1000; i++) {
            try {
                throw new BizException(DefaultErrorCode.SYS_ERROR);
            } catch (Exception e) {
                //System.out.println(e);
            }
        }
        System.out.println(System.currentTimeMillis()  - start);
        
         -> 3
         -> 13
         -> 71

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