【MySQL】使用JDBC时，无法连接远程MySQL数据库

使用JDBC时，无法连接远程MySQL数据库

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问题

今天连接MySQL的时候，程序很久没有反应，在过了几分钟后向我抛出了一堆异常：

警告: Having failed to acquire a resource, com.mchange.v2.resourcepool.BasicResourcePool@3d121db3 is interrupting all Threads waiting on a resource to check out. Will try again in response to new client requests.

java.sql.SQLException: Connections could not be acquired from the underlying database!

    at com.mchange.v2.sql.SqlUtils.toSQLException(SqlUtils.java:118)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:692)
    at com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource.getConnection(AbstractPoolBackedDataSource.java:140)
    at cn.wjinlong.spring.jdbc.JDBCTest.testDataSource(JDBCTest.java:43)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:51)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: com.mchange.v2.resourcepool.CannotAcquireResourceException: A ResourcePool could not acquire a resource from its primary factory or source.
    at com.mchange.v2.resourcepool.BasicResourcePool.awaitAvailable(BasicResourcePool.java:1469)
    at com.mchange.v2.resourcepool.BasicResourcePool.prelimCheckoutResource(BasicResourcePool.java:644)
    at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:554)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutAndMarkConnectionInUse(C3P0PooledConnectionPool.java:758)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:685)
    ... 24 more
Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
    at sun.reflect.GeneratedConstructorAccessor6.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
    at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:989)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:341)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2192)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2225)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2024)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:779)
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47)
    at sun.reflect.GeneratedConstructorAccessor5.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:389)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:330)
    at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:175)
    at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:220)
    at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:206)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:203)
    at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1138)
    at com.mchange.v2.resourcepool.BasicResourcePool.doAcquireAndDecrementPendingAcquiresWithinLockOnSuccess(BasicResourcePool.java:1125)
    at com.mchange.v2.resourcepool.BasicResourcePool.access$700(BasicResourcePool.java:44)
    at com.mchange.v2.resourcepool.BasicResourcePool$ScatteredAcquireTask.run(BasicResourcePool.java:1870)
    at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:696)
Caused by: java.net.ConnectException: Connection refused: connect
    at java.net.DualStackPlainSocketImpl.connect0(Native Method)
    at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:79)
    at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
    at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
    at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
    at java.net.Socket.connect(Socket.java:589)
    at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:211)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:300)
    ... 20 more

解决过程

异常的大概意思就是无法从数据库获得连接，所以：
1. 我首先去检查了数据库配置文件中的用户名、密码和url等信息，确保了自己没有手残地多打了几个空格，检查了几遍后，发现配置文件是没有问题的。
2. 在确保我的配置信息完全没有问题的情况下，我又想到可能我的数据库的用户权限可能存在问题，于是我去检查了用户的权限：
SQL语句为：SHOW GRANTS FOR 'test'@'%'; test为用户名，%为host。

可以看到，test用户拥有对test数据库的所有权限。
3. 这时候我就很懵逼了，想不到哪里出了问题，之前也连接过很多次，为啥会出问题呢？仔细想了想，这次和之前的唯一不同就是，这次连接的是远程的MySQL，之前连接的都是本机的MySQL，于是我就想到是不是远程的防火墙禁止外部访问呢？于是去查询了远程主机的防火墙状态：

可以看到，防火墙并没有拦截的规则。
4. 很气，哈哈哈。这真是奇怪啊，想了几遍，还是没有头绪，突然聪明的小脑瓜灵光一闪，我打算使用Navicat（图形化连接数据库工具）连接一下远程的数据库，看看到底是哪里的问题。

2003 - Can’t connect to MySQL server on ‘xx.xx.xx.xx’ (10061 “Unknown error”)

成功的报了错，我很开心，毕竟证明程序没有问题，而且有了错误编号，于是就去搜索相关问题。有人说这个错误可能是MySQL服务没有启动，吓得我瑟瑟发抖，这本应该是我首先想到的问题，赶紧去检查了下MySQL 的状态：

虚惊一场，我就说么，怎么会犯这种错误。
5. 没办法了，是时候祭出我的大法器啦——官方文档
从官网找到文档，又找到了连接错误出现的地方。点击连接可以直达。

从文档中发现一段话，和我的问题高度吻合。
于是我去查看了下MySQL的配置文件：

注意红框里的bind-address=127.0.0.1，这是MySQL的msyql默认设置。这时候我，们看一下MySQL的端口情况:

好像发现了些问题，注意我红框里的127.0.0.1，这里就是问题的原因。这个稍后再解释，我们先把bind-addrss=12.70.0.1注释掉，重启MySQL，再次连接，连接成功！！

这时候我们看一下MySQL的端口情况：

此时的Local Address已经从127.0.0.1–>0.0.0.0这就表示，从外部访问数据库时可以通过和本机的任何ip进行通信，而不是本地回环ip127.0.0.1。

总结

关于bind-address为127.0.0.1是MySQL的默认设置，这样的设置会阻止远程的数据库连接。所以当你要从远程访问MySQL的时候，再MySQL的配置文件中可以把这一行注释掉，或者把127.0.0.1换成你客户端的ip。