我正在努力让我的数据库与我的Java程序对话。

有人能给我一个使用JDBC的快速而简单的示例程序吗?

我得到了一个相当大的错误

Exception in thread "main" 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.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:409)
    at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1122)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2260)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:787)
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:49)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    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 java.sql.DriverManager.getConnection(DriverManager.java:582)
    at java.sql.DriverManager.getConnection(DriverManager.java:207)
    at SqlTest.main(SqlTest.java:22)
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.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:409)
    at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1122)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:344)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2181)
    ... 12 more
Caused by: java.net.ConnectException: Connection refused
    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:432)
    at java.net.Socket.connect(Socket.java:529)
    at java.net.Socket.connect(Socket.java:478)
    at java.net.Socket.<init>(Socket.java:375)
    at java.net.Socket.<init>(Socket.java:218)
    at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:256)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:293)
    ... 13 more

测试文件内容:

import com.mysql.jdbc.*;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class SqlTest {

    public static void main(String [] args) throws Exception {
        // Class.forName( "com.mysql.jdbc.Driver" ); // do this in init
        // // edit the jdbc url 
        Connection conn = DriverManager.getConnection( 
            "jdbc:mysql://localhost:3306/projects?user=user1&password=123");
        // Statement st = conn.createStatement();
        // ResultSet rs = st.executeQuery( "select * from table" );

        System.out.println("Connected?");
    }
}

当前回答

用于远程调用Mysql

Add remote user to Mysql from for exemple IP=remoteIP : mysql -u xxxx -p //local coonection to mysql mysql> GRANT ALL PRIVILEGES ON *.* TO 'theNewUser'@'remoteIP' IDENTIFIED BY 'passWord'; //Query OK, 0 rows affected (xx sec) mysql> FLUSH PRIVILEGES; //Query OK, 0 rows affected Allow remote access to Mysql (by default all externall call is not allowed): Edit /etc/mysql/mysql.conf.d/mysqld.cnf or /etc/mysql/my.cnf Change line: bind-address = 127.0.0.1 to bind-address = 0.0.0.0 Restart Mysql: /etc/init.d/mysql restart For The latest version of JDBC Driver, the JDBC : jdbc.url='jdbc:mysql://remoteIP:3306/yourDbInstance?autoReconnect=true&amp;useUnicode=true&amp;useJDBCCompliantTimezoneShift=true&amp;useLegacyDatetimeCode=false&amp;serverTimezone=UTC' jdbc.user='theNewUser'

其他回答

在MySQL Workbench中,有一种非常特殊的情况会发生这种问题,特别是在使用Eclipse JPA和本地主机下的持久性时。问题是,正如在其他一些回答中提到的,MySQL服务器在创建连接时可以非常严格地使用localhost, 0.0.0.0.1或127.0.0.1。为了克服这个问题,在(Windows)中的hosts文件:

C:\Windows\System32\drivers\etc

需要为主连接(例如myserver)添加以下条目:

#我的数据库服务器 127.0.0.1 myserver

如果没有添加到主机,那么每次尝试从127.0.0.1连接数据库时,连接都会失败

下载MySQL-JDBC-Type-4-Treiber (i.g。Mysql -connector-java-5.1.11-bin.jar从Mysql -connector-java-5.1.11.zip)

在编译和运行时期间,您需要在类路径中包含驱动程序jar。

Class.forName( "com.mysql.jdbc.Driver" ); // do this in init
// edit the jdbc url 
Connection conn = DriverManager.getConnection( "jdbc:mysql://MyDbComputerNameOrIP:3306/myDatabaseName", username, password );
Statement st = conn.createStatement();
ResultSet rs = st.executeQuery( "select * from table" );

我可能找错了对象,但您的异常似乎表明您的MySQL服务器不可用。

线程main中出现异常: 日志含义通信链路故障最后一个成功发送到服务器的报文为0 毫秒前。驱动未收到服务器的报文。处理步骤在…

如果你尝试(从终端)会发生什么?

mysql -u username -p

系统将提示您输入与用户名关联的密码。在你给出正确的密码后,mysql客户端连接吗?

如果没有,你可能需要从首选项启动MySQL。您还可以将其设置为在启动时运行。

当Java出堆时,我捕捉到这个异常。如果我试图在RAM中放入许多数据项-首先我捕捉到“通信链路故障”,然后是“OutOfMemoryError”。

我记录了它,我减少了内存消耗(删除1/2数据),一切都好。

这也是MySQL正在消亡的原因。在我的例子中,我有一个查询触发了MySQL中的一个错误,所以每当执行查询时,服务器就会死机并重新启动。查看MySQL错误日志,看看是否发生了这种情况。 在我的例子中(MacOS上的Homebrew),它位于/usr/local/var/mysql/<machinename>.err

因为我是在一个测试系统上看到的,所以我最终放弃并重新创建了我的数据库,这解决了问题,但我不敢想象如果它是在生产系统上,我会做什么。