Java链接数据库 #02# JDBC经典套路

时间 2019-12-02

标签 java 链接数据库 jdbc 经典套路栏目 Java 繁體版

原文原文链接

内容索引html

接Java链接数据库 #01# JDBC单线程适用，主要是改成多线程适用，顺便对DAO层结构进行改良：java

connection由共享变量改成由一个ThreadLocal变量保存
添加一个connection的管理类来控制事务
dao层统一贯上抛出DaoException
dao层对象改成单例

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① LocalConnectionFactory.java 负责建立connection/用ThreadLocal变量保存connectionlinux

package org.sample.db;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ResourceBundle;

public class LocalConnectionFactory {

    private LocalConnectionFactory() {
        // Exists to defeat instantiation
    }

    private static ResourceBundle rb = ResourceBundle.getBundle("org.sample.db.db-config");

    private static final String JDBC_URL = rb.getString("jdbc.url");

    private static final String JDBC_USER = rb.getString("jdbc.username");

    private static final String JDBC_PASSWORD = rb.getString("jdbc.password");

    private static final ThreadLocal<Connection> LocalConnectionHolder = new ThreadLocal<>();

    public static Connection getConnection() throws SQLException {
        Connection conn = LocalConnectionHolder.get();
        if (conn == null || conn.isClosed()) {
            conn = DriverManager.getConnection(JDBC_URL, JDBC_USER, JDBC_PASSWORD);
            LocalConnectionHolder.set(conn);
        }
        return conn;
    }

    public static void removeLocalConnection() {
        LocalConnectionHolder.remove();
    }
}

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② LocalConnectionProxy.java 经过该类间接控制connection，在Service层控制事务（代码分层有错误！）数据库

package org.sample.manager;

import org.sample.db.LocalConnectionFactory;
import org.sample.exception.DaoException;

import java.sql.Connection;
import java.sql.SQLException;

public class LocalConnectionProxy {

    public static void setAutoCommit(boolean autoCommit) throws DaoException {
        try {
            Connection conn = LocalConnectionFactory.getConnection();
            conn.setAutoCommit(autoCommit);
        } catch (SQLException e) {
            throw new DaoException(e);
        }
    }

//    public static void setTransactionIsolation(int level) throws DaoException {
//        try {
//            Connection conn = LocalConnectionFactory.getConnection();
//            conn.setTransactionIsolation(level);
//        } catch (SQLException e) {
//            throw new DaoException(e);
//        }
//    }

    public static void commit() throws DaoException {
        try {
            Connection conn = LocalConnectionFactory.getConnection();
            conn.commit();
        } catch (SQLException e) {
            throw new DaoException(e);
        }
    }

    public static void rollback() throws DaoException {
        try {
            Connection conn = LocalConnectionFactory.getConnection();
            conn.rollback();
        } catch (SQLException e) {
            throw new DaoException(e);
        }
    }

    public static void close() throws DaoException {
        try {
            Connection conn = LocalConnectionFactory.getConnection();
            conn.close();
            LocalConnectionFactory.removeLocalConnection();
        } catch (SQLException e) {
            throw new DaoException(e);
        }
    }
}

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③ ProfileDAO.java-2.0 标示出了可能抛出的异常缓存

package org.sample.dao;

import org.sample.entity.Profile;
import org.sample.exception.DaoException;

import java.util.List;

public interface ProfileDAO {

    int saveProfile(Profile profile);

    List<Profile> listProfileByNickname(String nickname);

    Profile getProfileByUsername(String username);

    int updateProfileById(Profile profile);

    int updatePassword(String username, String password);

    int updateLastOnline(String username);
}

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④ ProfileDAOImpl.java-2.0 改进了savePOJO方法，该类是线程安全的，而且适合采用单例模式。至于为何不直接用静态方法，在网上看到的是：静态方法通常都是业务无关的，而DAO方法都是业务相关的，而且建立对象能够方便框架（Spring）进行依赖注入，采用多态等。服务器

package org.sample.dao.impl;

import org.sample.dao.ProfileDAO;
import org.sample.db.LocalConnectionFactory;
import org.sample.entity.Profile;
import org.sample.exception.DaoException;
import org.sample.util.DbUtil;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

public class ProfileDAOImpl implements ProfileDAO {

    public static final ProfileDAO INSTANCE = new ProfileDAOImpl();

    private ProfileDAOImpl() {}

    @Override
    public int saveProfile(Profile profile) {
        int i = 0;
        try {
            Connection conn = LocalConnectionFactory.getConnection();
            String sql = "INSERT ignore INTO `profiles`.`profile` (`username`, `password`, `nickname`) " +
                    "VALUES (?, ?, ?)"; // 添加ignore出现重复不会抛出异常而是返回0
            try (PreparedStatement ps = conn.prepareStatement(sql)) {
                ps.setString(1, profile.getUsername());
                ps.setString(2, profile.getPassword());
                ps.setString(3, profile.getNickname());
                i = ps.executeUpdate();
            }
        } catch (SQLException e) {
            throw new DaoException(e);
        }
        return i;
    }

    @Override
    public List<Profile> listProfileByNickname(String nickname) {
        List<Profile> result = new ArrayList<>();
        try {
            Connection conn = LocalConnectionFactory.getConnection();
            String sql = "SELECT  `profile_id`,  `username`,  `password`,  `nickname`,  `last_online`,  `gender`,  `birthday`,  `location`,  `joined`" +
                    "FROM `profiles`.`profile`" +
                    "WHERE `nickname`=?";
            try (PreparedStatement ps = conn.prepareStatement(sql)) {
                ps.setString(1, nickname);
                try (ResultSet rs = ps.executeQuery()) {
                    while (rs.next()) {
                        Profile profile = DbUtil.extractProfileFromResultSet(rs);
                        result.add(profile);
                    }
                }
            }
        } catch (SQLException e) {
            throw new DaoException(e);
        }
        return result;
    }

    @Override
    public Profile getProfileByUsername(String username) {
        Profile result = null;
        try {
            Connection conn = LocalConnectionFactory.getConnection();
            String sql = "SELECT  `profile_id`,  `username`,  `password`,  `nickname`,  `last_online`,  `gender`,  `birthday`,  `location`,  `joined`" +
                    "FROM `profiles`.`profile`" +
                    "WHERE `username`=?";
            try (PreparedStatement ps = conn.prepareStatement(sql)) {
                ps.setString(1, username);
                try (ResultSet rs = ps.executeQuery()) {
                    if (rs.next()) {
                        result = DbUtil.extractProfileFromResultSet(rs);
                    }
                }
            }
        } catch (SQLException e) {
            throw new DaoException(e);
        }
        return result;
    }

    @Override
    public int updateProfileById(Profile profile) {
        int i = 0;
        try {
            Connection conn = LocalConnectionFactory.getConnection();
            String sql = "UPDATE `profiles`.`profile`" +
                    "SET `nickname`=?,  `gender`=?,  `birthday`=?,  `location`=? " +
                    "WHERE  `profile_id`=?";
            try (PreparedStatement ps = conn.prepareStatement(sql)) {
                ps.setString(1, profile.getNickname());
                ps.setString(2, profile.getGender() != null ? String.valueOf(profile.getGender()) : null);
                ps.setTimestamp(3, profile.getBirthday());
                ps.setString(4, profile.getLocation());
                ps.setLong(5, profile.getProfileId());
                i = ps.executeUpdate();
            }
        } catch (SQLException e) {
            throw new DaoException(e);
        }
        return i;
    }

    @Override
    public int updatePassword(String username, String password) {
        int i = 0;
        try {
            Connection conn = LocalConnectionFactory.getConnection();
            String sql = "UPDATE `profiles`.`profile`" +
                    "SET `password`=? " +
                    "WHERE  `username`=?";
            try (PreparedStatement ps = conn.prepareStatement(sql)) {
                ps.setString(1, password);
                ps.setString(2, username);
                i = ps.executeUpdate();
            }
        } catch (SQLException e) {
            throw new DaoException(e);
        }
        return i;
    }

    @Override
    public int updateLastOnline(String username) {
        int i = 0;
        try {
            Connection conn = LocalConnectionFactory.getConnection();
            String sql = "UPDATE `profiles`.`profile`" +
                    "SET `last_online`=CURRENT_TIMESTAMP " +
                    "WHERE  `username`=?";
            try (PreparedStatement ps = conn.prepareStatement(sql)) {
                ps.setString(1, username);
                i = ps.executeUpdate();
            }
        } catch (SQLException e) {
            throw new DaoException(e);
        }
        return i;
    }
}

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⑤ DaoException.java（RuntimeException）

package org.sample.exception;

public class DaoException extends OnesProfileException {

    public DaoException() {
        super();
    }

    public DaoException(String message) {
        super(message);
    }

    public DaoException(String message, Throwable cause) {
        super(message, cause);
    }

    public DaoException(Throwable cause) {
        super(cause);
    }

    protected DaoException(String message, Throwable cause, boolean enableSuppression, boolean writableStackTrace) {
        super(message, cause, enableSuppression, writableStackTrace);
    }
}

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⑥DbUtil.java-2.0 改成一个单纯的数据库操做辅助类。。。

package org.sample.util;

import org.sample.entity.Profile;

import java.sql.ResultSet;
import java.sql.SQLException;

public class DbUtil {

    public static Profile extractProfileFromResultSet(ResultSet rs) throws SQLException {
        Profile profile = new Profile();
        profile.setBirthday(rs.getTimestamp("birthday"));
        profile.setJoined(rs.getTimestamp("joined"));
        profile.setLast_online(rs.getTimestamp("last_online"));
        profile.setLocation(rs.getString("location"));
        profile.setNickname(rs.getString("nickname"));
        profile.setPassword(rs.getString("password"));
        profile.setProfileId(rs.getLong("profile_id"));
        profile.setUsername(rs.getString("username"));
        if (rs.getString("gender") != null) {
            profile.setGender(rs.getString("gender").charAt(0));
        }
        return profile;
    }
}

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⑦ ProfileDAOTest.java-2.0 各个方法测试之间再也不相互依赖

package org.sample.dao;

import org.junit.Test;
import org.sample.dao.impl.ProfileDAOImpl;
import org.sample.entity.Profile;
import org.sample.manager.LocalConnectionProxy;

import java.util.List;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertNotNull;

public class ProfileDAOTest {

    private static final ProfileDAO PROFILE_DAO = ProfileDAOImpl.INSTANCE;

    private static final String ORIGIN_STRING = "hello";
    private static final String PASSWORD = ORIGIN_STRING;

    private static String RandomString() {
        return Math.random() + ORIGIN_STRING + Math.random();
    }

    private static Profile RandomProfile() {
        Profile profile = new Profile(RandomString(), PASSWORD, RandomString());
        return profile;
    }

    @Test
    public void saveProfile() throws Exception {
        Profile profile = RandomProfile();
        int i = PROFILE_DAO.saveProfile(profile);
        int j = PROFILE_DAO.saveProfile(profile);
        LocalConnectionProxy.close();


        assertEquals(1, i);
        assertEquals(0, j);
    }

    @Test
    public void listProfileByNickname() throws Exception {
        final String nickName = RandomString();
        Profile profile1 = new Profile(RandomString(), PASSWORD, nickName);
        Profile profile2 = new Profile(RandomString(), PASSWORD, nickName);
        Profile profile3 = new Profile(RandomString(), PASSWORD, nickName);
        PROFILE_DAO.saveProfile(profile1);
        PROFILE_DAO.saveProfile(profile2);
        PROFILE_DAO.saveProfile(profile3);
        List result = PROFILE_DAO.listProfileByNickname(nickName);
        LocalConnectionProxy.close();

        assertEquals(3, result.size());
    }

    @Test
    public void getProfileByUsername() throws Exception {
        Profile profile = RandomProfile();
        PROFILE_DAO.saveProfile(profile);
        Profile result = PROFILE_DAO.getProfileByUsername(profile.getUsername());
        LocalConnectionProxy.close();

        assertNotNull(result);
    }

    @Test
    public void updateProfileById() throws Exception {
        Profile profile = RandomProfile();
        PROFILE_DAO.saveProfile(profile);
        Profile temp = PROFILE_DAO.getProfileByUsername(profile.getUsername());
        int i = PROFILE_DAO.updateProfileById(temp);

        LocalConnectionProxy.close();

        assertEquals(1, i);
    }

    @Test
    public void updatePassword() throws Exception {
        Profile profile = RandomProfile();
        PROFILE_DAO.saveProfile(profile);
        int i = PROFILE_DAO.updatePassword(profile.getUsername(), RandomString());
        LocalConnectionProxy.close();

        assertEquals(1, i);
    }

    @Test
    public void updateLastOnline() throws Exception {
        Profile profile = RandomProfile();
        PROFILE_DAO.saveProfile(profile);
        int i = PROFILE_DAO.updateLastOnline(profile.getUsername());
        LocalConnectionProxy.close();

        assertEquals(1, i);
    }

}

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⑧ DaoTest.java 简单测试了下在并发状况下代码的运行情况

package org.sample.manager;

import org.junit.Test;
import org.sample.dao.ProfileDAO;
import org.sample.dao.impl.ProfileDAOImpl;
import org.sample.entity.Profile;
import org.sample.exception.DaoException;

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.logging.Logger;

import static org.junit.Assert.assertTrue;

public class DaoTest {

    private static final Logger LOGGER = Logger.getLogger(DaoTest.class.getName());

    private static final String ORIGIN_STRING = "hello";
    private static String RandomString() {
        return Math.random() + ORIGIN_STRING + Math.random();
    }
    private static Profile RandomProfile() {
        Profile profile = new Profile(RandomString(), ORIGIN_STRING, RandomString());
        return profile;
    }

    private static final ProfileDAO PROFILE_DAO = ProfileDAOImpl.INSTANCE;

    private class Worker implements Runnable {
        private final Profile profile = RandomProfile();

        @Override
        public void run() {
            LOGGER.info(Thread.currentThread().getName() + " has started his work");
            try {
                LocalConnectionProxy.setAutoCommit(false);
                PROFILE_DAO.saveProfile(profile);
                LocalConnectionProxy.commit();
            } catch (DaoException e) {
                e.printStackTrace();
            } finally {
                try {
                    LocalConnectionProxy.close();
                } catch (DaoException e) {
                    e.printStackTrace();
                }
            }
            LOGGER.info(Thread.currentThread().getName() + " has finished his work");
        }
    }

    private static final int numTasks = 100;

    @Test
    public void test() throws Exception {
        List<Runnable> workers = new LinkedList<>();
        for(int i = 0; i != numTasks; ++i) {
            workers.add(new Worker());
        }
        assertConcurrent("Dao test ", workers, Integer.MAX_VALUE);
    }

    public static void assertConcurrent(final String message, final List<? extends Runnable> runnables, final int maxTimeoutSeconds) throws InterruptedException {
        final int numThreads = runnables.size();
        final List<Throwable> exceptions = Collections.synchronizedList(new ArrayList<Throwable>());
        final ExecutorService threadPool = Executors.newFixedThreadPool(numThreads);
        try {
            final CountDownLatch allExecutorThreadsReady = new CountDownLatch(numThreads);
            final CountDownLatch afterInitBlocker = new CountDownLatch(1);
            final CountDownLatch allDone = new CountDownLatch(numThreads);
            for (final Runnable submittedTestRunnable : runnables) {
                threadPool.submit(new Runnable() {
                    public void run() {
                        allExecutorThreadsReady.countDown();
                        try {
                            afterInitBlocker.await();
                            submittedTestRunnable.run();
                        } catch (final Throwable e) {
                            exceptions.add(e);
                        } finally {
                            allDone.countDown();
                        }
                    }
                });
            }
            // wait until all threads are ready
            assertTrue("Timeout initializing threads! Perform long lasting initializations before passing runnables to assertConcurrent", allExecutorThreadsReady.await(runnables.size() * 10, TimeUnit.MILLISECONDS));
            // start all test runners
            afterInitBlocker.countDown();
            assertTrue(message +" timeout! More than" + maxTimeoutSeconds + "seconds", allDone.await(maxTimeoutSeconds, TimeUnit.SECONDS));
        } finally {
            threadPool.shutdownNow();
        }
        assertTrue(message + "failed with exception(s)" + exceptions, exceptions.isEmpty());
    }
}

在链接数量超出100时会抛出 com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Data source rejected establishment of connection, message from server: "Too many connections" ，这是由于超出了mysql默认的最大链接数，这个值是能够自定义的，应根据具体的并发量、服务器性能、业务场景等各类因素综合决定。参考链接池最大/最小链接数，自动释放时间，设置多少合适？

★查看数据库链接相关信息：

mysql> show status like 'Threads%';
+-------------------+-------+
| Variable_name     | Value |
+-------------------+-------+
| Threads_cached    | 8     |
| Threads_connected | 1     |
| Threads_created   | 3419  |
| Threads_running   | 1     |
+-------------------+-------+
4 rows in set (0.00 sec)

Threads_cached是数据库缓存的链接数。

★查看最大链接数：

mysql> show variables like '%max_connections%';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| max_connections | 100   |
+-----------------+-------+
1 row in set (0.01 sec)

mysql> show global status like 'Max_used_connections';
 ps. 官方解释 - The maximum number of connections that have been in use simultaneously since the server started. +----------------------+-------+
| Variable_name        | Value |
+----------------------+-------+
| Max_used_connections | 101   |
+----------------------+-------+
1 row in set (0.00 sec)

可是我很奇怪为何max_used_connections会大于max_connections，查文档结果以下 ↓

mysqld actually permits max_connections+1 clients to connect. The extra connection is reserved for use by accounts that have theCONNECTION_ADMIN or SUPER privilege. By granting the SUPERprivilege to administrators and not to normal users (who should not need it), an administrator can connect to the server and use SHOW PROCESSLIST to diagnose problems even if the maximum number of unprivileged clients are connected. See Section 13.7.6.29, “SHOW PROCESSLIST Syntax”.

顺便mark 10 MySQL variables that you should monitor - TechRepublic

后续修改↓

一、简化dao层方法命名

二、更改LocalConnectionFactory.java，依然采用Class.forName

package org.sample.db;

import org.sample.exception.DaoException;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ResourceBundle;

public class LocalConnectionFactory {

    private LocalConnectionFactory() {
        // Exists to defeat instantiation
    }

    private static ResourceBundle rb = ResourceBundle.getBundle("org.sample.db.db-config");

    private static final String JDBC_URL = rb.getString("jdbc.url");

    private static final String JDBC_USER = rb.getString("jdbc.username");

    private static final String JDBC_PASSWORD = rb.getString("jdbc.password");

    private static final ThreadLocal<Connection> LocalConnectionHolder = new ThreadLocal<>();

    static {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            // 虽说JDBC4以后已经再也不须要Class.forName，可是可否
            // 自动注册和环境、版本的相关性很大，因此安全起见仍是加上这句比较好。
        } catch (ClassNotFoundException e) {
            // TODO 日志
            throw new DaoException("could not register JDBC driver", e);
        }
    }

    public static Connection getConnection() throws SQLException {
        Connection conn = LocalConnectionHolder.get();
        if (conn == null || conn.isClosed()) {
            conn = DriverManager.getConnection(JDBC_URL, JDBC_USER, JDBC_PASSWORD);
            LocalConnectionHolder.set(conn);
        }
        return conn;
    }

    public static void removeLocalConnection() {
        // TODO 应该先关掉再remove?
        LocalConnectionHolder.remove();
    }
}

LocalConnectionFactory.java

三、补一张结构草图（WRONG！！！）：