juc之常用4大并发工具类 (四)

CountDownLatch: 减少计数
CyclicBarrier: 循环栅栏
Semaphore: 信号量
ExChanger: 交换器
文章目录
- - - 1.CountDownLatch
    - 2.CyclicBarrier
    - 3.Semaphore
    - 4.Exchanger

1.CountDownLatch

CountDownLatch,俗称闭锁,作用是类似加强版的 Join,是让一组线程等待其他的线程完成工作以后才执行

就比如在启动框架服务的时候,我们主线程需要在环境线程初始化完成之后才能启动,这时候我们就可以实现使用 CountDownLatch 来完成

public CountDownLatch(int count) {
    if (count < 0) throw new IllegalArgumentException("count < 0");
    this.sync = new Sync(count);
}

传入一个 int 类型的参数,将决定在执行次扣减之后,等待的线程被唤醒

在这里插入图片描述

主线程中创建 CountDownLatch(3),然后主线程 await 阻塞,然后线程 A,B,C 各自完成了任务,调用了 countDown,之后,每个线程调用一次计数器就会减一,初始是 3,然后 A 线程调用后变成 2,B 线程调用后变成 1,C 线程调用后,变成 0,这时就会唤醒正在 await 的主线程,然后主线程继续执行

线程休眠辅助工具类

package org.dance.tools;

import java.util.concurrent.TimeUnit;

/**
 * 类说明：线程休眠辅助工具类
 */
public class SleepTools {

    /**
     * 按秒休眠
     *
     * @param seconds 秒数
     */
    public static final void second(int seconds) {
        try {
            TimeUnit.SECONDS.sleep(seconds);
        } catch (InterruptedException e) {
        }
    }

    /**
     * 按毫秒数休眠
     *
     * @param seconds 毫秒数
     */
    public static final void ms(int seconds) {
        try {
            TimeUnit.MILLISECONDS.sleep(seconds);
        } catch (InterruptedException e) {
        }
    }
}

package org.dance.day2.util;

import org.dance.tools.SleepTools;

import java.util.concurrent.CountDownLatch;

/**
 * CountDownLatch的使用,有五个线程,6个扣除点
 * 扣除完成后主线程和业务线程,才能执行工作
 *  扣除点一般都是大于等于需要初始化的线程的
 */
public class UseCountDownLatch {

    /**
     * 设置为6个扣除点
     */
    static CountDownLatch countDownLatch = new CountDownLatch(6);

    /**
     * 初始化线程
     */
    private static class InitThread implements Runnable {

        @Override
        public void run() {

            System.out.println("thread_" + Thread.currentThread().getId() + " ready init work .....");

            // 执行扣减 扣减不代表结束
            countDownLatch.countDown();

            for (int i = 0; i < 2; i++) {
                System.out.println("thread_" + Thread.currentThread().getId() + ".....continue do its work");
            }

        }
    }

    /**
     * 业务线程
     */
    private static class BusiThread implements Runnable {

        @Override
        public void run() {

            // 业务线程需要在等初始化完毕后才能执行
            try {
                countDownLatch.await();
                for (int i = 0; i < 3; i++) {
                    System.out.println("BusiThread " + Thread.currentThread().getId() + " do business-----");
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {

        // 创建单独的初始化线程
        new Thread(){
            @Override
            public void run() {
                SleepTools.ms(1);
                System.out.println("thread_" + Thread.currentThread().getId() + " ready init work step 1st.....");
                // 扣减一次
                countDownLatch.countDown();
                System.out.println("begin stop 2nd.....");
                SleepTools.ms(1);
                System.out.println("thread_" + Thread.currentThread().getId() + " ready init work step 2nd.....");
                // 扣减一次
                countDownLatch.countDown();

            }
        }.start();
        // 启动业务线程
        new Thread(new BusiThread()).start();
        // 启动初始化线程
        for (int i = 0; i <= 3; i++) {
            new Thread(new InitThread()).start();
        }
        // 主线程进入等待
        try {
            countDownLatch.await();
            System.out.println("Main do ites work.....");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }

}

thread_13 ready init work .....
thread_13.....continue do its work
thread_13.....continue do its work
thread_14 ready init work .....
thread_14.....continue do its work
thread_14.....continue do its work
thread_15 ready init work .....
thread_15.....continue do its work
thread_11 ready init work step 1st.....
begin stop 2nd.....
thread_16 ready init work .....
thread_16.....continue do its work
thread_16.....continue do its work
thread_15.....continue do its work
thread_11 ready init work step 2nd.....
Main do ites work.....
BusiThread 12 do business-----
BusiThread 12 do business-----
BusiThread 12 do business-----

2.CyclicBarrier

CyclicBarrier,俗称栅栏锁,作用是让一组线程到达某个屏障,被阻塞,一直到组内的最后一个线程到达,然后屏障开放,接着,所有的线程继续运行

public CyclicBarrier(int parties) {
    this(parties, null);
}

public class Demo {
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }
}

第一个参数也是 Int 类型的,传入的是执行线程的个数,这个数量和 CountDownLatch 不一样,这个数量是需要和线程数量吻合的,CountDownLatch 则不一样,CountDownLatch 可以大于等于,而 CyclicBarrier 只能等于,然后是第二个参数,第二个参数是 barrierAction,这个参数是当屏障开放后,执行的任务线程,如果当屏障开放后需要执行什么任务,可以写在这个线程中

在这里插入图片描述

主线程创建 CyclicBarrier(3,barrierAction),然后由线程开始执行,线程 A,B 执行完成后都调用了 await,然后他们都在一个屏障前阻塞者,需要等待线程 C 也,执行完成,调用 await 之后,然后三个线程都达到屏障后,屏障开放,然后线程继续执行,并且 barrierAction 在屏障开放的一瞬间也开始执行

public class UseCyclicBarrier {

    /**
     * 存放子线程工作结果的安全容器
     */
    private static ConcurrentHashMap<String, Long> resultMap = new ConcurrentHashMap<>();

    private static CyclicBarrier cyclicBarrier = new CyclicBarrier(5,new CollectThread());

    /**
     * 结果打印线程
     * 用来演示CyclicBarrier的第二个参数,barrierAction
     */
    private static class CollectThread implements Runnable {

        @Override
        public void run() {
            StringBuffer result = new StringBuffer();
            for (Map.Entry<String, Long> workResult : resultMap.entrySet()) {
                result.append("[" + workResult.getValue() + "]");
            }
            System.out.println("the result = " + result);
            System.out.println("do other business.....");

        }
    }

    /**
     * 工作子线程
     * 用于CyclicBarrier的一组线程
     */
    private static class SubThread implements Runnable {

        @Override
        public void run() {
            // 获取当前线程的ID
            long id = Thread.currentThread().getId();
            // 放入统计容器中
            resultMap.put(String.valueOf(id), id);
            Random random = new Random();
            try {
                if (random.nextBoolean()) {
                    Thread.sleep(1000 + id);
                    System.out.println("Thread_"+id+"..... do something");
                }
                System.out.println(id+" is await");
                cyclicBarrier.await();
                Thread.sleep(1000+id);
                System.out.println("Thread_"+id+".....do its business");
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (BrokenBarrierException e) {
                e.printStackTrace();
            }

        }
    }

    public static void main(String[] args) {
        for (int i = 0; i <= 4; i++) {
            Thread thread = new Thread(new SubThread());
            thread.start();
        }
    }

}

11 is await
14 is await
15 is await
Thread_12..... do something
12 is await
Thread_13..... do something
13 is await
the result = [11][12][13][14][15]
do other business.....
Thread_11.....do its business
Thread_12.....do its business
Thread_13.....do its business
Thread_14.....do its business
Thread_15.....do its business

在这里插入图片描述

3.Semaphore

Semaphore,俗称信号量,作用于控制同时访问某个特定资源的线程数量,用在流量控制

一说特定资源控制,那么第一时间就想到了数据库连接…

之前用等待超时模式写了一个数据库连接池,打算用这个 Semaphone 也写一个

public Semaphore(int permits) {
    sync = new NonfairSync(permits);
}

需要传入许可证的数量,这个数量就是资源的最大允许的访问的线程数

基于Semaphore 的数据库连接池

public class SqlConnection implements Connection {

    /**
     * 获取数据库连接
     * @return
     */
    public static final Connection fetchConnection(){
        return new SqlConnection();
    }

    @Override
    public void commit() throws SQLException {
        SleepTools.ms(70);
    }

    @Override
    public Statement createStatement() throws SQLException {
        SleepTools.ms(1);
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql) throws SQLException {
        return null;
    }

    @Override
    public CallableStatement prepareCall(String sql) throws SQLException {
        return null;
    }

    @Override
    public String nativeSQL(String sql) throws SQLException {
        return null;
    }

    @Override
    public void setAutoCommit(boolean autoCommit) throws SQLException {

    }

    @Override
    public boolean getAutoCommit() throws SQLException {
        return false;
    }

    @Override
    public void rollback() throws SQLException {

    }

    @Override
    public void close() throws SQLException {

    }

    @Override
    public boolean isClosed() throws SQLException {
        return false;
    }

    @Override
    public DatabaseMetaData getMetaData() throws SQLException {
        return null;
    }

    @Override
    public void setReadOnly(boolean readOnly) throws SQLException {

    }

    @Override
    public boolean isReadOnly() throws SQLException {
        return false;
    }

    @Override
    public void setCatalog(String catalog) throws SQLException {

    }

    @Override
    public String getCatalog() throws SQLException {
        return null;
    }

    @Override
    public void setTransactionIsolation(int level) throws SQLException {

    }

    @Override
    public int getTransactionIsolation() throws SQLException {
        return 0;
    }

    @Override
    public SQLWarning getWarnings() throws SQLException {
        return null;
    }

    @Override
    public void clearWarnings() throws SQLException {

    }

    @Override
    public Statement createStatement(int resultSetType, int resultSetConcurrency) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency) throws SQLException {
        return null;
    }

    @Override
    public CallableStatement prepareCall(String sql, int resultSetType, int resultSetConcurrency) throws SQLException {
        return null;
    }

    @Override
    public Map<String, Class<?>> getTypeMap() throws SQLException {
        return null;
    }

    @Override
    public void setTypeMap(Map<String, Class<?>> map) throws SQLException {

    }

    @Override
    public void setHoldability(int holdability) throws SQLException {

    }

    @Override
    public int getHoldability() throws SQLException {
        return 0;
    }

    @Override
    public Savepoint setSavepoint() throws SQLException {
        return null;
    }

    @Override
    public Savepoint setSavepoint(String name) throws SQLException {
        return null;
    }

    @Override
    public void rollback(Savepoint savepoint) throws SQLException {

    }

    @Override
    public void releaseSavepoint(Savepoint savepoint) throws SQLException {

    }

    @Override
    public Statement createStatement(int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
        return null;
    }

    @Override
    public CallableStatement prepareCall(String sql, int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int autoGeneratedKeys) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int[] columnIndexes) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, String[] columnNames) throws SQLException {
        return null;
    }

    @Override
    public Clob createClob() throws SQLException {
        return null;
    }

    @Override
    public Blob createBlob() throws SQLException {
        return null;
    }

    @Override
    public NClob createNClob() throws SQLException {
        return null;
    }

    @Override
    public SQLXML createSQLXML() throws SQLException {
        return null;
    }

    @Override
    public boolean isValid(int timeout) throws SQLException {
        return false;
    }

    @Override
    public void setClientInfo(String name, String value) throws SQLClientInfoException {

    }

    @Override
    public void setClientInfo(Properties properties) throws SQLClientInfoException {

    }

    @Override
    public String getClientInfo(String name) throws SQLException {
        return null;
    }

    @Override
    public Properties getClientInfo() throws SQLException {
        return null;
    }

    @Override
    public Array createArrayOf(String typeName, Object[] elements) throws SQLException {
        return null;
    }

    @Override
    public Struct createStruct(String typeName, Object[] attributes) throws SQLException {
        return null;
    }

    @Override
    public void setSchema(String schema) throws SQLException {

    }

    @Override
    public String getSchema() throws SQLException {
        return null;
    }

    @Override
    public void abort(Executor executor) throws SQLException {

    }

    @Override
    public void setNetworkTimeout(Executor executor, int milliseconds) throws SQLException {

    }

    @Override
    public int getNetworkTimeout() throws SQLException {
        return 0;
    }

    @Override
    public <T> T unwrap(Class<T> iface) throws SQLException {
        return null;
    }

    @Override
    public boolean isWrapperFor(Class<?> iface) throws SQLException {
        return false;
    }
}

连接池对象

public class DBPoolSemaphore {

    /**
     * 池容量
     */
    private final static int POOL_SIZE = 10;

    /**
     * useful 代表可用连接
     * useless 代表已用连接
     *  为什么要使用两个Semaphore呢?是因为,在连接池中不只有连接本身是资源,空位也是资源,也需要记录
     */
    private final Semaphore useful, useless;

    /**
     * 连接池
     */
    private final static LinkedList<Connection> POOL = new LinkedList<>();

    /**
     * 使用静态块初始化池
     */
    static {
        for (int i = 0; i < POOL_SIZE; i++) {
            POOL.addLast(SqlConnection.fetchConnection());
        }
    }

    public DBPoolSemaphore() {
        // 初始可用的许可证等于池容量
        useful = new Semaphore(POOL_SIZE);
        // 初始不可用的许可证容量为0
        useless = new Semaphore(0);
    }

    /**
     * 获取数据库连接
     *
     * @return 连接对象
     */
    public Connection takeConnection() throws InterruptedException {
        // 可用许可证减一
        useful.acquire();
        Connection connection;
        synchronized (POOL) {
            connection = POOL.removeFirst();
        }
        // 不可用许可证数量加一
        useless.release();
        return connection;
    }

    /**
     * 释放链接
     *
     * @param connection 连接对象
     */
    public void returnConnection(Connection connection) throws InterruptedException {
        if(null!=connection){
            // 打印日志
            System.out.println("当前有"+useful.getQueueLength()+"个线程等待获取连接,,"
                    +"可用连接有"+useful.availablePermits()+"个");
            // 不可用许可证减一
            useless.acquire();
            synchronized (POOL){
                POOL.addLast(connection);
            }
            // 可用许可证加一
            useful.release();
        }
    }

}

测试

public class UseSemaphore {

    /**
     * 连接池
     */
    public static final DBPoolSemaphore pool = new DBPoolSemaphore();

    private static class BusiThread extends Thread{
        @Override
        public void run() {
            // 随机数工具类 为了让每个线程持有连接的时间不一样
            Random random = new Random();
            long start = System.currentTimeMillis();
            try {
                Connection connection = pool.takeConnection();
                System.out.println("Thread_"+Thread.currentThread().getId()+
                        "_获取数据库连接耗时["+(System.currentTimeMillis()-start)+"]ms.");
                // 模拟使用连接查询数据
                SleepTools.ms(100+random.nextInt(100));
                System.out.println("查询数据完成归还连接");
                pool.returnConnection(connection);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        for (int i = 0; i < 50; i++) {
            BusiThread busiThread = new BusiThread();
            busiThread.start();
        }
    }

}

Thread_11_获取数据库连接耗时[0]ms.
Thread_12_获取数据库连接耗时[0]ms.
Thread_13_获取数据库连接耗时[0]ms.
Thread_14_获取数据库连接耗时[0]ms.
Thread_15_获取数据库连接耗时[0]ms.
Thread_16_获取数据库连接耗时[0]ms.
Thread_17_获取数据库连接耗时[0]ms.
Thread_18_获取数据库连接耗时[0]ms.
Thread_19_获取数据库连接耗时[0]ms.
Thread_20_获取数据库连接耗时[0]ms.
查询数据完成归还连接
当前有40个线程等待获取连接,,可用连接有0个
Thread_21_获取数据库连接耗时[112]ms.
查询数据完成归还连接
...................查询数据完成归还连接
当前有2个线程等待获取连接,,可用连接有0个
Thread_59_获取数据库连接耗时[637]ms.
查询数据完成归还连接
当前有1个线程等待获取连接,,可用连接有0个
Thread_60_获取数据库连接耗时[660]ms.
查询数据完成归还连接
当前有0个线程等待获取连接,,可用连接有0个
查询数据完成归还连接...................
当前有0个线程等待获取连接,,可用连接有8个
查询数据完成归还连接
当前有0个线程等待获取连接,,可用连接有9个

对连接池中的资源的到了控制,这就是信号量的流量控制。

4.Exchanger

Exchanger,俗称交换器,用于在线程之间交换数据,但是比较受限,因为只能两个线程之间交换数据

public Exchanger() {
    participant = new Participant();
}

public class UseExchange {

    private static final Exchanger<Set<String>> exchanger = new Exchanger<>();

    public static void main(String[] args) {

        new Thread(){
            @Override
            public void run() {
                Set<String> aSet = new HashSet<>();
                aSet.add("A");
                aSet.add("B");
                aSet.add("C");
                try {
                    Set<String> exchange = exchanger.exchange(aSet);
                    for (String s : exchange) {
                        System.out.println("aSet"+s);
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }.start();

        new Thread(){
            @Override
            public void run() {
                Set<String> bSet = new HashSet<>();
                bSet.add("1");
                bSet.add("2");
                bSet.add("3");
                try {
                    Set<String> exchange = exchanger.exchange(bSet);
                    for (String s : exchange) {
                        System.out.println("bSet"+s);
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }.start();

    }

}