JDBC連接不上數據庫

今天晚上出現了一個特別鬱悶的問題，一直測試得好好的項目竟然運行不了，原因在連接不上數據庫，爲了解決這個問題，折騰了很久，而且讓我很窩火，不過，最後問題被我們老師發現了，也就解決。
爲了讓自己下次不再犯這樣的錯，現在詳細解決這個問題

1.當數據庫沒安裝時，會出現的錯誤：

com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception: 

** BEGIN NESTED EXCEPTION ** 

java.net.ConnectException
MESSAGE: Connection refused: connect

STACKTRACE:

java.net.ConnectException: Connection refused: connect
	at java.net.PlainSocketImpl.socketConnect(Native Method)
	at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:333)
	at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:195)
	at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:182)
	at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366)
	at java.net.Socket.connect(Socket.java:519)
	at java.net.Socket.connect(Socket.java:469)
	at java.net.Socket.<init>(Socket.java:366)
	at java.net.Socket.<init>(Socket.java:209)
	at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:173)
	at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:267)
	at com.mysql.jdbc.Connection.createNewIO(Connection.java:2739)
	at com.mysql.jdbc.Connection.<init>(Connection.java:1553)
	at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:266)
	at java.sql.DriverManager.getConnection(DriverManager.java:582)
	at java.sql.DriverManager.getConnection(DriverManager.java:185)
	at DBConn.getConnection(DBConn.java:24)
	at DBConn.main(DBConn.java:43)

解決方法：安裝上數據庫就行
2.JDBC驅動的用戶名出錯

java.sql.SQLException: Access denied for user 'roote'@'localhost' (using password: YES)
	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:946)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2941)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:868)
	at com.mysql.jdbc.MysqlIO.secureAuth411(MysqlIO.java:3340)
	at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1238)
	at com.mysql.jdbc.Connection.createNewIO(Connection.java:2743)
	at com.mysql.jdbc.Connection.<init>(Connection.java:1553)
	at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:266)
	at java.sql.DriverManager.getConnection(DriverManager.java:582)
	at java.sql.DriverManager.getConnection(DriverManager.java:185)
	at DBConn.getConnection(DBConn.java:24)
	at DBConn.main(DBConn.java:43)

解決方法：改正用戶名
3.JDBC密碼出錯
出錯現象和2相同
解決方法：把密碼改正
4.沒引入驅動包

       java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
	at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:276)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
	at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:169)
	at DBConn.getConnection(DBConn.java:20)
	at DBConn.main(DBConn.java:43)

解決方法：引入驅動包
5.引入的驅動包有錯誤
解決方法：重新引入驅動包（我今天晚上就是遇到這個問題了，但是具體報錯現象沒記錄）

前車之鑑，後事之師！希望自己以後解決問題的能力越來越強