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CHAPTER 3 MAKING DATABASE CONNECTIONS
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* @param query a sql query to test against database connection * @return true if a given Connection object is a valid one; * otherwise return false. */ public static boolean testConnection(Connection conn, String query) { ResultSet rs = null; Statement stmt = null; try { stmt = conn.createStatement(); if (stmt == null) { return false; } rs = stmt.executeQuery(query); if (rs == null) { return false; } if (rs.next()) { // Connection object is valid: you were able to // connect to the database and return something useful. return true; } // there is no hope any more for the validity // of the Connection object return false; } catch(Exception e) { // something went wrong: connection is bad return false; } finally { DatabaseUtil.close(rs); DatabaseUtil.close(stmt); } } public static void main(String[] args) { Connection oracleConn = null; Connection mysqlConn = null; try { System.out.println("-- TestValidityOfConnection begin --"); // get connection to an Oracle database oracleConn = getOracleConnection(); System.out.println("oracleConn="+oracleConn); System.out.println(isValidConnection(oracleConn, "oracle")); // get connection to a MySQL database mysqlConn = getMySqlConnection(); System.out.println("mysqlConn="+mysqlConn); System.out.println(isValidConnection(mysqlConn, "mysql"));
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CHAPTER 3 MAKING DATABASE CONNECTIONS
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System.out.println("databases are shutting down..."); // sleep for 30 seconds (enough time to shut down // both Oracle and MySQL databases) and during this // time shut down both Oracle and MySQL databases: Thread.sleep(30000); // wake up after 30 seconds // test to see if the Oracle connection is valid System.out.println("oracleConn="+oracleConn); System.out.println(isValidConnection(oracleConn, "oracle")); // test to see if the MySQL connection is valid System.out.println("mysqlConn="+mysqlConn); System.out.println(isValidConnection(mysqlConn, "mysql")); System.out.println("-- TestValidityOfConnection end --"); } catch(Exception e){ // handle the exception e.printStackTrace(); System.exit(1); } finally { // release database resources DatabaseUtil.close(oracleConn); DatabaseUtil.close(mysqlConn); } } }
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Running Program: Testing Validity of Connection
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Use this code to test the validity of the connection: $ javac TestValidityOfConnection.java $ java TestValidityOfConnection -- TestValidityOfConnection begin -oracleConn=oracle.jdbc.driver.OracleConnection@1a125f0 true mysqlConn=com.mysql.jdbc.Connection@1372a1a true databases are shutting down... oracleConn=oracle.jdbc.driver.OracleConnection@1a125f0 false mysqlConn=com.mysql.jdbc.Connection@1372a1a false -- TestValidityOfConnection end -java.sql.SQLException: Io exception: Connection reset by peer: socket write error at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:134) at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:179) at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:333) at oracle.jdbc.driver.OracleConnection.close(OracleConnection.java:1442) at jcb.db.DatabaseUtil.close(DatabaseUtil.java:67) at TestValidityOfConnection.main(TestValidityOfConnection.java:176)
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CHAPTER 3 MAKING DATABASE CONNECTIONS
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3-21. How Do You Keep the Connection Alive in a Production Environment
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Maybe you have heard the following from a software/database engineer: I have a database application that works fine for a day and then stops working overnight.... This can happen to many database applications. In general, when you get a Connection object from database server, there is a timeout property associated with the Connection object; when this time expires, then the Connection object becomes stale/defunct (becoming a dead, useless Connection object). For example, the MySQL database closes connections after eight hours of inactivity. So, you either need to use a connection pool that handles stale connections or use the autoReconnect parameter. Using the MySQL database, automatic reconnection is available. Because the Connection object has to ping the database before each query, this is turned off by default. To use it, you need to pass autoReconnect=true in the connection URL. You may also change the number of reconnect tries and the initial timeout value via the parameters maxReconnects=n (the default is 3) and initialTimeout=n (the default is two seconds). The timeout is an exponential back-off timeout; in other words, if you have initial timeout of two seconds and a maxReconnects of three seconds, then the driver will timeout for two seconds, four seconds, and then sixteen seconds between each reconnection attempt.
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Solution: Creating Connection with Properties (MySQL)
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The following code shows how to keep connections alive in a production environment: import java.sql.*; import java.util.*; import jcb.util.DatabaseUtil; public class TestCreateConnectionWithProperties_MySQL { public public public public static static static static final final final final String String String String DATABASE_USER = "user"; DATABASE_PASSWORD = "password"; MYSQL_AUTO_RECONNECT = "autoReconnect"; MYSQL_MAX_RECONNECTS = "maxReconnects";
/** * Create MySQL connection...which will live for a long time */ public static Connection getConnection() throws Exception { String driver = "org.gjt.mm.mysql.Driver"; // load the driver Class.forName(driver); String dbURL = "jdbc:mysql://localhost/tiger"; String dbUsername = "root"; String dbPassword = "root"; // these are properties that get passed // to DriverManager.getConnection(...) java.util.Properties connProperties = new java.util.Properties(); connProperties.put(DATABASE_USER, dbUsername); connProperties.put(DATABASE_PASSWORD, dbPassword); // set additional connection properties: // if connection stales, then make automatically // reconnect; make it alive again; // if connection stales, then try for reconnection; connProperties.put(MYSQL_AUTO_RECONNECT, "true");
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