在使用个人阿里云测试机，在查询实时输出日志时，看到数据库连接失败后，服务器一直在重连服务器。开始以为是遭受重复攻击，后面把服务重启后，就没有出现一直重连的情况。看以下输出日志：

2022-02-09 11:04:58.896 ERROR 16876 --- [eate-1550991149] com.alibaba.druid.pool.DruidDataSource   : create connection SQLException, url: jdbc:mysql://47.98.67,98:1234/test?useSSL=false&characterEncoding=UTF-8&serverTimezone=UTC, errorCode 1045, state 28000java.sql.SQLException: Access denied for user 'root'@'113.90.123.76' (using password: YES)    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129) ~[mysql-connector-java-8.0.16.jar:8.0.16]    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97) ~[mysql-connector-java-8.0.16.jar:8.0.16]    at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122) ~[mysql-connector-java-8.0.16.jar:8.0.16]    at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:835) ~[mysql-connector-java-8.0.16.jar:8.0.16]    at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:455) ~[mysql-connector-java-8.0.16.jar:8.0.16]    at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:240) ~[mysql-connector-java-8.0.16.jar:8.0.16]    at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:199) ~[mysql-connector-java-8.0.16.jar:8.0.16]    at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:156) ~[druid-1.1.10.jar:1.1.10]    at com.alibaba.druid.filter.stat.StatFilter.connection_connect(StatFilter.java:218) ~[druid-1.1.10.jar:1.1.10]    at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:150) ~[druid-1.1.10.jar:1.1.10]    at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1560) ~[druid-1.1.10.jar:1.1.10]    at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1623) ~[druid-1.1.10.jar:1.1.10]    at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2468) ~[druid-1.1.10.jar:1.1.10]

注意上面一直有 druid 数据库连接池的提示，这里就想到可能是 druid 连接池的问题，然后去掉 druid maven 依赖后在请求接口就不会出现重连的问题。

druid 重连原因

在上图源码找到最后一行 DruidDataSource.java:2468 定位到源码上，CreateConnectionThread 创建连接线程，看一下 CreateConnectionThread 源码：

public class CreateConnectionThread extends Thread {        public CreateConnectionThread(String name){            super(name);            this.setDaemon(true);        }        public void run() {            initedLatch.countDown();            long lastDiscardCount = 0;            int errorCount = 0;            for (;;) {                // addLast                try {                    lock.lockInterruptibly();                } catch (InterruptedException e2) {                    break;                }                long discardCount = DruidDataSource.this.discardCount;                boolean discardChanged = discardCount - lastDiscardCount > 0;                lastDiscardCount = discardCount;                try {                    boolean emptyWait = true;                    if (createError != null                            && poolingCount == 0                            && !discardChanged) {                        emptyWait = false;                    }                    if (emptyWait                            && asyncInit && createCount < initialSize) {                        emptyWait = false;                    }                    if (emptyWait) {                        // 必须存在线程等待，才创建连接                        if (poolingCount >= notEmptyWaitThreadCount //                                && !(keepAlive && activeCount + poolingCount < minIdle)) {                            empty.await();                        }                        // 防止创建超过maxActive数量的连接                        if (activeCount + poolingCount >= maxActive) {                            empty.await();                            continue;                        }                    }                } catch (InterruptedException e) {                    lastCreateError = e;                    lastErrorTimeMillis = System.currentTimeMillis();                    if (!closing) {                        LOG.error("create connection Thread Interrupted, url: " + jdbcUrl, e);                    }                    break;                } finally {                    lock.unlock();                }                PhysicalConnectionInfo connection = null;                try {                    connection = createPhysicalConnection();                    setFailContinuous(false);                } catch (SQLException e) {                    LOG.error("create connection SQLException, url: " + jdbcUrl + ", errorCode " + e.getErrorCode()                              + ", state " + e.getSQLState(), e);                    errorCount++;                    if (errorCount > connectionErrorRetryAttempts && timeBetweenConnectErrorMillis > 0) {                        // fail over retry attempts                        setFailContinuous(true);                        if (failFast) {                            lock.lock();                            try {                                notEmpty.signalAll();                            } finally {                                lock.unlock();                            }                        }                        if (breakAfterAcquireFailure) {                            break;                        }                        try {                            Thread.sleep(timeBetweenConnectErrorMillis);                        } catch (InterruptedException interruptEx) {                            break;                        }                    }                } catch (RuntimeException e) {                    LOG.error("create connection RuntimeException", e);                    setFailContinuous(true);                    continue;                } catch (Error e) {                    LOG.error("create connection Error", e);                    setFailContinuous(true);                    break;                }                if (connection == null) {                    continue;                }                boolean result = put(connection);                if (!result) {                    JdbcUtils.close(connection.getPhysicalConnection());                    LOG.info("put physical connection to pool failed.");                }                errorCount = 0; // reset errorCount            }        }    }

这是一个多线程的类，而 run 方法里面设置了没有限制的 for 循环 for (;;) {}, 而日志报错定位的信息:

connection = createPhysicalConnection();

如果符合条件 errorCount > connectionErrorRetryAttempts && timeBetweenConnectErrorMillis > 0 会再次尝试重连，先看一下这几个参数的含义：

errorCount 错误次数

在 run 方法初始化时为零，每次连接失败，会自动加1

connectionErrorRetryAttempts

连接错误重试次数，默认值为 1。

protected int  connectionErrorRetryAttempts  = 1;

连接间隔时间，单位毫秒。默认值为 500。

protected volatile long timeBetweenConnectErrorMillis = DEFAULT_TIME_BETWEEN_CONNECT_ERROR_MILLIS;public static final long DEFAULT_TIME_BETWEEN_CONNECT_ERROR_MILLIS = 500;

我们在连接数据库失败后，要不在里面 break 中断，其中有

if (breakAfterAcquireFailure) {     break;}

将改 break-after-acquire-failure 设置成 true,在 application.properties 文件如下配置：

spring.datasource.druid.break-after-acquire-failure=true

如果想多尝试连接几次，需要设置 connection-error-retry-attempts ,当 errorCount 大于 connectionErrorRetryAttempts 才会进入到 条件内，才会中断循环。在 application.properties 文件如下配置：

spring.datasource.druid.connection-error-retry-attempts=3

druid 数据库连接失败，是因为在使用多线程连接数据时使用了无限制循环连接，需要在连接失败中断连接，需要设置 break-after-acquire-failure 为 true。设置之后数据库连接不成功也不会不断的重试。如果要设置重连次数要设置 connection-error-retry-attempts。