当我使用个人阿里云测试机时，在查看实时输出日志时，我发现数据库连接失败后，服务器一直在尝试重新连接。一开始认为系统遭受了连续的攻击，但是在重新启动服务后，就不再出现不断重连的情况了。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.(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;
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;
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。下面是 application.properties 文件的配置：
spring.datasource.druid.connection-error-retry-attempts=3
