1. 打开项目的页面如下：
可以从tomcat log中发现Connection has been abandoned PooledConnection和Too many connections。
2017-08-03 18:28:48.022 WARN 6396 --- [o-8080-exec-170] o.a.tomcat.jdbc.pool.ConnectionPool : Connection has been abandoned PooledConnection[com.mysql.jdbc.JDBC4Connection@af6bbd]:java.lang.Exception at org.apache.tomcat.jdbc.pool.ConnectionPool.getThreadDump(ConnectionPool.java:1065) at org.apache.tomcat.jdbc.pool.ConnectionPool.createConnection(ConnectionPool.java:707) at org.apache.tomcat.jdbc.pool.ConnectionPool.borrowConnection(ConnectionPool.java:634) at org.apache.tomcat.jdbc.pool.ConnectionPool.init(ConnectionPool.java:488) at org.apache.tomcat.jdbc.pool.ConnectionPool.<init>(ConnectionPool.java:144) at org.apache.tomcat.jdbc.pool.DataSourceProxy.pCreatePool(DataSourceProxy.java:116) at org.apache.tomcat.jdbc.pool.DataSourceProxy.createPool(DataSourceProxy.java:103) at org.apache.tomcat.jdbc.pool.DataSourceProxy.getConnection(DataSourceProxy.java:127) ...... at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:521) at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1096) at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:674) at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1500) at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1456) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:745) 2017-08-03 18:28:48 ERROR [com.xxx.yyy.a.b] -- <Too many connections> com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Too many connections at sun.reflect.GeneratedConstructorAccessor295.newInstance(Unknown Source) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) at java.lang.reflect.Constructor.newInstance(Constructor.java:422) at com.mysql.jdbc.Util.handleNewInstance(Util.java:409) at com.mysql.jdbc.Util.getInstance(Util.java:384) at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1015) at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3562) at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3494) at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:920) at com.mysql.jdbc.MysqlIO.secureAuth411(MysqlIO.java:4000) at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1285) at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2186) at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:787) at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:49) at sun.reflect.GeneratedConstructorAccessor288.newInstance(Unknown Source) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) at java.lang.reflect.Constructor.newInstance(Constructor.java:422) at com.mysql.jdbc.Util.handleNewInstance(Util.java:409) at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:357) at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:285) at org.apache.tomcat.jdbc.pool.PooledConnection.connectUsingDriver(PooledConnection.java:278) at org.apache.tomcat.jdbc.pool.PooledConnection.connect(PooledConnection.java:182) at org.apache.tomcat.jdbc.pool.ConnectionPool.createConnection(ConnectionPool.java:702) at org.apache.tomcat.jdbc.pool.ConnectionPool.borrowConnection(ConnectionPool.java:634) at org.apache.tomcat.jdbc.pool.ConnectionPool.init(ConnectionPool.java:488) at org.apache.tomcat.jdbc.pool.ConnectionPool.<init>(ConnectionPool.java:144) at org.apache.tomcat.jdbc.pool.DataSourceProxy.pCreatePool(DataSourceProxy.java:116) at org.apache.tomcat.jdbc.pool.DataSourceProxy.createPool(DataSourceProxy.java:103) at org.apache.tomcat.jdbc.pool.DataSourceProxy.getConnection(DataSourceProxy.java:127) ...... at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:521) at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1096) at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:674) at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1500) at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1456) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:745)
2. 解决方案
重启一下Tomcat或者Eclipse都OK，重新跑一下项目即可（Ctrl+Alt+D）。引起这个问题得主要原因是修改代码保存时Eclipse会自动重新编译代码，而此时你又修改代码再保存，就很容易出现这个HTTP 403错误。