使用 Apache Tomcat JDBC 的 JDBC 池单例数据库类

Question

我正在尝试配置一个单例数据库类，以允许使用 JDK8 和 Tomcat7 对 Oracle XE 18c 数据库进行连接池。

由于以下错误，我无法编译源代码：

未报告的异常 SQLException;必须被抓获或被宣布为 扔了

类源代码：

package com.example.webapp.db;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import org.apache.tomcat.jdbc.pool.DataSource;
import org.apache.tomcat.jdbc.pool.PoolProperties;
public class DatabaseManager {

    private static final DatabaseManager SINGLE_INSTANCE = new DatabaseManager();

    private DatabaseManager() throws SQLException {
        PoolProperties p = new PoolProperties();
        p.setUrl("jdbc:oracle:thin:@localhost:1521:xe");
        p.setDriverClassName("oracle.jdbc.driver.OracleDriver");
        p.setUsername("scott");
        p.setPassword("tiger");
        p.setJmxEnabled(true);
        p.setTestWhileIdle(false);
        p.setTestOnBorrow(true);
        p.setValidationQuery("SELECT 1");
        p.setTestOnReturn(false);
        p.setValidationInterval(30000);
        p.setTimeBetweenEvictionRunsMillis(30000);
        p.setMaxActive(100);
        p.setInitialSize(10);
        p.setMaxWait(10000);
        p.setRemoveAbandonedTimeout(60);
        p.setMinEvictableIdleTimeMillis(30000);
        p.setMinIdle(10);
        p.setLogAbandoned(true);
        p.setRemoveAbandoned(true);
        p.setJdbcInterceptors("org.apache.tomcat.jdbc.pool.interceptor.ConnectionState;"+
          "org.apache.tomcat.jdbc.pool.interceptor.StatementFinalizer");
        DataSource datasource = new DataSource();
        datasource.setPoolProperties(p); 

        Connection con = null;
        try {
          con = datasource.getConnection();
          Statement st = con.createStatement();
          ResultSet rs = st.executeQuery("select * from user");
          int cnt = 1;
          while (rs.next()) {
              System.out.println((cnt++)+". Host:" +rs.getString("Host")+
                " User:"+rs.getString("User")+" Password:"+rs.getString("Password"));
          }
          rs.close();
          st.close();
        } finally {
          if (con!=null) try {con.close();}catch (Exception ignore) {}
        }
    }

    public static DatabaseManager getInstance() {
        return SINGLE_INSTANCE;
    }
}

Answer 1

我认为您应该在 getInstance() 中抛出或处理 SQLException 并使用 try-with 资源（自动关闭资源）而不是 finally 块。您应该不使用会引发 Exception 进行初始化的构造函数将 instance 设为常量。

package com.example.webapp.db;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import org.apache.tomcat.jdbc.pool.DataSource;
import org.apache.tomcat.jdbc.pool.PoolProperties;

public class DatabaseManager {

    // not final anymore and null as default
    private static DatabaseManager instance = null;

    private DatabaseManager() {
        PoolProperties p = new PoolProperties();
        p.setUrl("jdbc:oracle:thin:@localhost:1521:xe");
        p.setDriverClassName("oracle.jdbc.driver.OracleDriver");
        p.setUsername("scott");
        p.setPassword("tiger");
        p.setJmxEnabled(true);
        p.setTestWhileIdle(false);
        p.setTestOnBorrow(true);
        p.setValidationQuery("SELECT 1");
        p.setTestOnReturn(false);
        p.setValidationInterval(30000);
        p.setTimeBetweenEvictionRunsMillis(30000);
        p.setMaxActive(100);
        p.setInitialSize(10);
        p.setMaxWait(10000);
        p.setRemoveAbandonedTimeout(60);
        p.setMinEvictableIdleTimeMillis(30000);
        p.setMinIdle(10);
        p.setLogAbandoned(true);
        p.setRemoveAbandoned(true);
        p.setJdbcInterceptors("org.apache.tomcat.jdbc.pool.interceptor.ConnectionState;"
                + "org.apache.tomcat.jdbc.pool.interceptor.StatementFinalizer");
        javax.sql.DataSource datasource = new DataSource();
        datasource.setPoolProperties(p);

        // use a try-with resource to get rid of the finally block...
        try (Connection con = datasource.getConnection()) {
            Statement st = con.createStatement();
            ResultSet rs = st.executeQuery("select * from user");
            int cnt = 1;

            while (rs.next()) {
                System.out.println((cnt++) + ". Host:" + rs.getString("Host")
                        + " User:" + rs.getString("User")
                        + " Password:" + rs.getString("Password"));
            }

            rs.close();
            st.close();
        // ... and handle the exception
        } catch (SQLException e) {
            System.err.println("SQLException while constructing the instance of DatabaseManager");
            e.printStackTrace();
        }
    }

    public static DatabaseManager getInstance() {
        // check for null here:
        if (instance == null) {
            instance = new DatabaseManager();
        }
        return instance;
    }
}

也许，为数据库连接创建一个初始化方法比在构造函数中初始化所有内容更好，但这是基于意见的。

Answer 2

改代码如下图，你要捕获异常。

private static  DatabaseManager SINGLE_INSTANCE = null;
static {
    try {
        SINGLE_INSTANCE = new DatabaseManager();
    }
    catch(Exception e) {
        e.printStackTrace();
    }
}