多线程的等待超时机制–wait(remain)

开发人员经常会遇到这样的方法调用场景：调用一个方法时等待一段时间（一般来说是给定一个时间段），如果该方法能够在给定的时间段之内得到结果，那么将结果立刻返回，反之，超时返回默认结果。等待/通知的经典范式，即加锁、条件循环和处理逻辑3个步骤，而这种范式无法做到超时等待。而超时等待的加入，只需要对经典范式做出非常小的改动，改动内容如下所示。

假设超时时间段是mills，那么可以推断出在当前时间System.currentTimeMillis()+mills之后就会超时。
long overtime = System.currentTimeMillis()+mills;
long remain = mills;

这时仅需要wait(remain)即可，在wait(remain)返回之后会将执行：
remain = overtime - System.currentTimeMillis(); 
如果remain小于等于0，表示已经超时，直接退出，否则将继续执行wait(remain)。

数据库连接池示例：

数据库连接池的设计也可以复用到其他的资源获取的场景，针对昂贵资源（比如数据库连接）的获取都应该加以超时限制。

数据库连接的实现：

public class SqlConnectImpl implements Connection {

    /*拿一个数据库连接*/
    public static final Connection fetchConnection() {
        return new SqlConnectImpl();
    }

    @Override
    public boolean isWrapperFor(Class<?> arg0) throws SQLException {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public <T> T unwrap(Class<T> arg0) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public void abort(Executor arg0) throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public void clearWarnings() throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public void close() throws SQLException {
        // TODO Auto-generated method stub

    }

    @SneakyThrows
    @Override
    public void commit() throws SQLException {
        TimeUnit.MILLISECONDS.sleep(70);
    }

    @Override
    public Array createArrayOf(String arg0, Object[] arg1) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Blob createBlob() throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Clob createClob() throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public NClob createNClob() throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public SQLXML createSQLXML() throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @SneakyThrows
    @Override
    public Statement createStatement() throws SQLException {
        TimeUnit.MILLISECONDS.sleep(1);
        return null;
    }

    @Override
    public Statement createStatement(int arg0, int arg1) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Statement createStatement(int arg0, int arg1, int arg2) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Struct createStruct(String arg0, Object[] arg1) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public boolean getAutoCommit() throws SQLException {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public String getCatalog() throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Properties getClientInfo() throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public String getClientInfo(String arg0) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public int getHoldability() throws SQLException {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public DatabaseMetaData getMetaData() throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public int getNetworkTimeout() throws SQLException {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public String getSchema() throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public int getTransactionIsolation() throws SQLException {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public Map<String, Class<?>> getTypeMap() throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public SQLWarning getWarnings() throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public boolean isClosed() throws SQLException {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public boolean isReadOnly() throws SQLException {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public boolean isValid(int arg0) throws SQLException {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public String nativeSQL(String arg0) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public CallableStatement prepareCall(String arg0) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public CallableStatement prepareCall(String arg0, int arg1, int arg2) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public CallableStatement prepareCall(String arg0, int arg1, int arg2, int arg3) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String arg0) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String arg0, int arg1) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String arg0, int[] arg1) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String arg0, String[] arg1) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String arg0, int arg1, int arg2) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String arg0, int arg1, int arg2, int arg3) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public void releaseSavepoint(Savepoint arg0) throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public void rollback() throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public void rollback(Savepoint arg0) throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public void setAutoCommit(boolean arg0) throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public void setCatalog(String arg0) throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public void setClientInfo(Properties arg0) throws SQLClientInfoException {
        // TODO Auto-generated method stub

    }

    @Override
    public void setClientInfo(String arg0, String arg1) throws SQLClientInfoException {
        // TODO Auto-generated method stub

    }

    @Override
    public void setHoldability(int arg0) throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public void setNetworkTimeout(Executor arg0, int arg1) throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public void setReadOnly(boolean arg0) throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public Savepoint setSavepoint() throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Savepoint setSavepoint(String arg0) throws SQLException {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public void setSchema(String arg0) throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public void setTransactionIsolation(int arg0) throws SQLException {
        // TODO Auto-generated method stub

    }

    @Override
    public void setTypeMap(Map<String, Class<?>> arg0) throws SQLException {
        // TODO Auto-generated method stub

    }

}

数据库的连接池的一个简单实现：

public class DBPool {

    //数据库池的容器
    private static LinkedList<Connection> pool = new LinkedList<>();

	//初始化线程池
    public DBPool(int initalSize) {
        if (initalSize > 0) {
            for (int i = 0; i < initalSize; i++) {
                pool.addLast(SqlConnectImpl.fetchConnection());
            }
        }
    }

    //在mills时间内还拿不到数据库连接，返回一个null
    public Connection fetchConn(long mills) throws InterruptedException {
        synchronized (pool) {
            if (mills < 0) {
                while (pool.isEmpty()) {
                    pool.wait();
                }
                return pool.removeFirst();
            } else {
                long overtime = System.currentTimeMillis() + mills;
                long remain = mills;
                while (pool.isEmpty() && remain > 0) {
                    pool.wait(remain);
                    remain = overtime - System.currentTimeMillis();
                }
                Connection result = null;
                if (!pool.isEmpty()) {
                    result = pool.removeFirst();
                }
                return result;
            }
        }
    }

    //放回数据库连接
    public void releaseConn(Connection conn) {
        if (conn != null) {
            synchronized (pool) {
                pool.addLast(conn);
                pool.notifyAll();  //唤醒其他线程可以来获取释放的线程
            }
        }
    }
    
}

测试类：

public class DBPoolTest {
    static DBPool pool = new DBPool(10);
    // 控制器:控制main线程将会等待所有Woker结束后才能继续执行
    static CountDownLatch end;

    public static void main(String[] args) throws Exception {
        // 线程数量
        int threadCount = 50;
        end = new CountDownLatch(threadCount);
        int count = 20;//每个线程的操作次数
        AtomicInteger got = new AtomicInteger();//计数器：统计可以拿到连接的线程
        AtomicInteger notGot = new AtomicInteger();//计数器：统计没有拿到连接的线程
        for (int i = 0; i < threadCount; i++) {
            Thread thread = new Thread(new Worker(count, got, notGot),
                    "worker_" + i);
            thread.start();
        }
        end.await();// main线程在此处等待
        System.out.println("总共尝试了: " + (threadCount * count));
        System.out.println("拿到连接的次数：  " + got);
        System.out.println("没能连接的次数： " + notGot);
    }

    static class Worker implements Runnable {
        int count;
        AtomicInteger got;
        AtomicInteger notGot;

        public Worker(int count, AtomicInteger got,
                      AtomicInteger notGot) {
            this.count = count;
            this.got = got;
            this.notGot = notGot;
        }

        public void run() {
            while (count > 0) {
                try {
                    // 从线程池中获取连接，如果1000ms内无法获取到，将会返回null
                    // 分别统计连接获取的数量got和未获取到的数量notGot
                    Connection connection = pool.fetchConn(1000);
                    if (connection != null) {
                        try {
                            connection.createStatement();
                            connection.commit();
                        } finally {
                            pool.releaseConn(connection);
                            got.incrementAndGet();
                        }
                    } else {
                        notGot.incrementAndGet();
                        System.out.println(Thread.currentThread().getName()
                                + "等待超时!");
                    }
                } catch (Exception ex) {
                } finally {
                    count--;
                }
            }
            end.countDown();
        }
    }
}

运行结果：

原文链接：https://blog.csdn.net/qq_38658642/article/details/107443215

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