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 28000
java.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]
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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 } } }
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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();
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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;
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timeBetweenConnectErrorMillis
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;
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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;}
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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
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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
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总结
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。
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