JavaWeb之事务&数据库连接池

Posted 2021-01-26

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1. 事务

Transaction 其实指的就是一组操作，里面包含许多单一的逻辑，如果有一个逻辑没有执行成功，那么

个事务就是执行失败，所有的数据都会回滚到未执行前的状态。

事务是为解决数据安全操作提出的，事务控制实际上就是控制数据的安全访问，比如银行转账。

2. 事务的使用

命令行方式

# 开启事务
start transaction;
# 提交事务
commit;
# 回滚事务
rollback;

代码方式

import org.junit.Test;

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

public class Demo01 {
    @Test
    public void transactionDemo(){
        Connection conn = null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        try {
            conn = ConnectionUtil.getConnection();
            //关闭事务自动提交
            conn.setAutoCommit(false);
            String sql = "select * from account";
            pstmt = conn.prepareStatement(sql);
            rs = pstmt.executeQuery();
            while (rs.next()){
                System.out.println(rs.getString("name") + "==" + rs.getInt("money"));
            }
            //所有操作执行完成后手动的提交一下
            conn.commit();
        } catch (SQLException e) {
            e.printStackTrace();
            try {
                //回滚事务
                conn.rollback();
            } catch (SQLException e1) {
                e1.printStackTrace();
            }
        } finally {
            ConnectionUtil.release(rs, pstmt, conn);
        }

    }
}

3. 事务特性(ACID)

原子性事务中包含的逻辑不可再分
一致性事务执行前后，数据的完整性保持一直
隔离性事务在执行期间，不能受到其他事务的影响
持久性事务执行成功，应该持久化到磁盘上

4. 安全问题

读
- 脏读
在A窗口中设置隔离级别为读未提交，在A、B两个窗口中开启事务，在B窗口中修改数据。在A中可以查询到B窗口中还未提交的数据。一个事务中读取到另一个事务还未提交的数据，就是脏读。读到的是数据库内存中的数据，并非磁盘上真正的数据。
- 不可以重复读
在A窗口中设置隔离级别为读已提交，在A、B两个窗口中开启事务，在B窗口中修改数据。在A中就不可以查询到B窗口还未提交的数据了，这样就解决了脏读的问题，但是这样会引发一个新的问题，那就是只能读取到已经提交的数据。这样的话，前后读取到的结果是不一致的，发生了不可重复读，所谓不可重复度，就是不能执行多次读取，否则会出现查询结果不一致。

将隔离级别设置为重复读，就可以解决整个问题了。
- 幻读
一个事务读取到另一个事务已插入的数据，导致查询结果不一致。

将隔离级别设置为可串行化，就可以解决这些问题了，到底可串行化是怎么解决这个问题的呢？

在A窗口中设置隔离级别为可串行化，在A、B两个窗口中开启事务，在B窗口中修改数据，只有B窗口执行commit，A窗口才可以查询数据。这个级别一般比较少用，因为它的效率比较低。
写
- 丢失更新
两个不同的事务在某一时刻对同一数据执行修改操作，导致第一次操作数据丢失
- 乐观锁
  
  l乐观锁认为事务不一定会产生丢失更新，让事务进行并发修改，不对事务进行锁定。发现并发修改某行数据时，乐观锁抛出异常。让用户解决。可以通过给数据表添加自增的version字段进行数据修改时，数据库会检测version字段和事务中的version字段是否一致。若不一致，抛出异常，交给程序猿自己处理。
- 悲观锁
  
  悲观锁认为一定会发生丢失更新，所以悲观锁要求一个事务执行提交之后，其他事务才能查询修改数据。

5. 隔离级别

Read Uncommitted 读未提交，引发脏读问题
Read Committed 读已提交，解决脏读，引发不可重复读问题(Oracle默认隔离级别)
Repeatable Read 重复读，解决不可重复读，未解决幻读(mysql默认隔离级别)
Serializable，可串行化解决所有问题

隔离级别分类

按性能从高到低可划分为：读未提交>读已提交>重复读>可串行化
按拦截程序从高到低可划分为：可串行化>重复读>读已提交>读未提交

6. 数据库连接池

数据库在使用的时候再去创建连接，这是一件非常耗时的操作，为了改善用户体验，我们可以在程序开始的时候，在内存中开辟一块空间，称为数据库连接池，一开始往池子里放多个连接对象，如果有用户需要使用数据库连接，就从池子里取对象，当操作完成后将连接归还，这样就可以做到连接复用

自定义数据库连接池

import javax.sql.DataSource;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.SQLException;
import java.sql.SQLFeatureNotSupportedException;
import java.util.ArrayList;
import java.util.List;
import java.util.logging.Logger;

/**
 * 数据库连接池
 */
public class MyDataSource implements DataSource {

    List<Connection> dataSoucePool = new ArrayList<Connection>();

    public MyDataSource() {
        for (int i = 0; i < 10;i++){
            //将10个连接放到连接池中
            Connection conn = ConnectionUtil.getConnection();
            dataSoucePool.add(conn);
        }
    }

    @Override
    public Connection getConnection() throws SQLException {
        if (dataSoucePool.size() == 0){
            //如果连接池已经没有空闲的连接了，扩容
            for (int i = 0; i < 5;i++){
                Connection conn = ConnectionUtil.getConnection();
                dataSoucePool.add(conn);
            }
        }
        //每次都移出连接池第一个连接对象
        Connection conn = dataSoucePool.remove(0);
        Connection connection = new ConnectionWrap(conn,dataSoucePool);
        return connection;
    }

    public Connection backConnectuon(Connection conn){
        dataSourcePool.add(conn);        
    }

    @Override
    public Connection getConnection(String username, String password) throws SQLException {
        return null;
    }

    @Override
    public <T> T unwrap(Class<T> iface) throws SQLException {
        return null;
    }

    @Override
    public boolean isWrapperFor(Class<?> iface) throws SQLException {
        return false;
    }

    @Override
    public PrintWriter getLogWriter() throws SQLException {
        return null;
    }

    @Override
    public void setLogWriter(PrintWriter out) throws SQLException {

    }

    @Override
    public void setLoginTimeout(int seconds) throws SQLException {

    }

    @Override
    public int getLoginTimeout() throws SQLException {
        return 0;
    }

    @Override
    public Logger getParentLogger() throws SQLFeatureNotSupportedException {
        return null;
    }
}

解决自定义连接池出现的问题

自定义连接池出现了什么问题呢？
自定义连接池多增加了一个backConnection方法来归还连接，违背了面向接口编程的规范。我们可以使用装饰者模式来包装Connection类，以符合面向接口的规范。
import java.sql.*;
import java.util.List;
import java.util.Map;
import java.util.Properties;
import java.util.concurrent.Executor;

/**
 * 包装Connection类
 */
public class ConnectionWrap implements java.sql.Connection {
    Connection connection = null;
    List<Connection> list = null;

    public ConnectionWrap(Connection connection, List<Connection> list) {
        this.connection = connection;
        this.list = list;
    }

    @Override
    public Statement createStatement() throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql) throws SQLException {
        return connection.prepareStatement(sql);
    }

    @Override
    public CallableStatement prepareCall(String sql) throws SQLException {
        return null;
    }

    @Override
    public String nativeSQL(String sql) throws SQLException {
        return null;
    }

    @Override
    public void setAutoCommit(boolean autoCommit) throws SQLException {

    }

    @Override
    public boolean getAutoCommit() throws SQLException {
        return false;
    }

    @Override
    public void commit() throws SQLException {

    }

    @Override
    public void rollback() throws SQLException {

    }

    @Override
    public void close() throws SQLException {
        System.out.println(list.size());
        list.add(connection);
        System.out.println(list.size());
    }

    @Override
    public boolean isClosed() throws SQLException {
        return false;
    }

    @Override
    public DatabaseMetaData getMetaData() throws SQLException {
        return null;
    }

    @Override
    public void setReadOnly(boolean readOnly) throws SQLException {

    }

    @Override
    public boolean isReadOnly() throws SQLException {
        return false;
    }

    @Override
    public void setCatalog(String catalog) throws SQLException {

    }

    @Override
    public String getCatalog() throws SQLException {
        return null;
    }

    @Override
    public void setTransactionIsolation(int level) throws SQLException {

    }

    @Override
    public int getTransactionIsolation() throws SQLException {
        return 0;
    }

    @Override
    public SQLWarning getWarnings() throws SQLException {
        return null;
    }

    @Override
    public void clearWarnings() throws SQLException {

    }

    @Override
    public Statement createStatement(int resultSetType, int resultSetConcurrency) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency) throws SQLException {
        return null;
    }

    @Override
    public CallableStatement prepareCall(String sql, int resultSetType, int resultSetConcurrency) throws SQLException {
        return null;
    }

    @Override
    public Map<String, Class<?>> getTypeMap() throws SQLException {
        return null;
    }

    @Override
    public void setTypeMap(Map<String, Class<?>> map) throws SQLException {

    }

    @Override
    public void setHoldability(int holdability) throws SQLException {

    }

    @Override
    public int getHoldability() throws SQLException {
        return 0;
    }

    @Override
    public Savepoint setSavepoint() throws SQLException {
        return null;
    }

    @Override
    public Savepoint setSavepoint(String name) throws SQLException {
        return null;
    }

    @Override
    public void rollback(Savepoint savepoint) throws SQLException {

    }

    @Override
    public void releaseSavepoint(Savepoint savepoint) throws SQLException {

    }

    @Override
    public Statement createStatement(int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
        return null;
    }

    @Override
    public CallableStatement prepareCall(String sql, int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int autoGeneratedKeys) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int[] columnIndexes) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, String[] columnNames) throws SQLException {
        return null;
    }

    @Override
    public Clob createClob() throws SQLException {
        return null;
    }

    @Override
    public Blob createBlob() throws SQLException {
        return null;
    }

    @Override
    public NClob createNClob() throws SQLException {
        return null;
    }

    @Override
    public SQLXML createSQLXML() throws SQLException {
        return null;
    }

    @Override
    public boolean isValid(int timeout) throws SQLException {
        return false;
    }

    @Override
    public void setClientInfo(String name, String value) throws SQLClientInfoException {

    }

    @Override
    public void setClientInfo(Properties properties) throws SQLClientInfoException {

    }

    @Override
    public String getClientInfo(String name) throws SQLException {
        return null;
    }

    @Override
    public Properties getClientInfo() throws SQLException {
        return null;
    }

    @Override
    public Array createArrayOf(String typeName, Object[] elements) throws SQLException {
        return null;
    }

    @Override
    public Struct createStruct(String typeName, Object[] attributes) throws SQLException {
        return null;
    }

    @Override
    public void setSchema(String schema) throws SQLException {

    }

    @Override
    public String getSchema() throws SQLException {
        return null;
    }

    @Override
    public void abort(Executor executor) throws SQLException {

    }

    @Override
    public void setNetworkTimeout(Executor executor, int milliseconds) throws SQLException {

    }

    @Override
    public int getNetworkTimeout() throws SQLException {
        return 0;
    }

    @Override
    public <T> T unwrap(Class<T> iface) throws SQLException {
        return null;
    }

    @Override
    public boolean isWrapperFor(Class<?> iface) throws SQLException {
        return false;
    }
}

7. 常用开源数据库连接池有哪些？

DBCP

# 不使用配置文件方式
public void demo(){
        BasicDataSource basicDataSource = new BasicDataSource();
        basicDataSource.setDriverClassName("com.mysql.jdbc.Driver");
        basicDataSource.setUrl("jdbc:mysql://localhost:3306/bank");
        basicDataSource.setUsername("root");
        basicDataSource.setPassword("123456");
        Connection connection = null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        try{
            connection = basicDataSource.getConnection();
            String sql = "select * from account";
            pstmt = connection.prepareStatement(sql);
            rs = pstmt.executeQuery();
            while (rs.next()){
                System.out.println(rs.getString("name") + rs.getInt("money"));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            ConnectionUtil.release(rs, pstmt, connection);
        }
    }
# 使用配置文件方式
public void demo()  {

        BasicDataSourceFactory factory = new BasicDataSourceFactory();
        Properties properties = new Properties();
        Connection connection = null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        try {
            InputStream inputStream = new FileInputStream("src\dbcpconfig.properties");
            properties.load(inputStream);
            DataSource dataSource = factory.createDataSource(properties);
            connection = dataSource.getConnection();
            String sql = "select * from account";
            pstmt = connection.prepareStatement(sql);
            rs = pstmt.executeQuery();
            while (rs.next()){
                System.out.println(rs.getString("name") + rs.getInt("money"));
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            ConnectionUtil.release(rs, pstmt, connection);
        }

    }

C3P0

# 不使用配置文件方式
public void demo(){
        ComboPooledDataSource dataSource = new ComboPooledDataSource();
        try {
            dataSource.setDriverClass("com.mysql.jdbc.Driver");
        } catch (PropertyVetoException e) {
            e.printStackTrace();
        }
        dataSource.setJdbcUrl("jdbc:mysql://localhost:3306/bank");
        dataSource.setUser("root");
        dataSource.setPassword("123456");
        Connection connection = null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        try{
            connection = dataSource.getConnection();
            String sql = "select * from account";
            pstmt = connection.prepareStatement(sql);
            rs = pstmt.executeQuery();
            while (rs.next()){
                System.out.println(rs.getString("name") + rs.getInt("money"));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            ConnectionUtil.release(rs, pstmt, connection);
        }
    }
# 使用配置文件方式
public void demo()  {

        ComboPooledDataSource dataSource = new ComboPooledDataSource();
        Connection connection = null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        try {
            connection = dataSource.getConnection();
            String sql = "select * from account";
            pstmt = connection.prepareStatement(sql);
            rs = pstmt.executeQuery();
            while (rs.next()){
                System.out.println(rs.getString("name") + rs.getInt("money"));
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            ConnectionUtil.release(rs, pstmt, connection);
        }
    }

8. DbUtils

增删改

QueryRunner queryRunner = new QueryRunner(new ComboPooledDataSource());
//增加
queryRunner.update("insert into account values (null , ? , ? )", "aa" ,1000);
//删除
queryRunner.update("delete from account where id = ?", 5);
//更新
queryRunner.update("update account set money = ? where id = ?", 10000000 , 6);

查询

直接new接口的匿名实现类

public void demo(){
      QueryRunner queryRunner = new QueryRunner(new ComboPooledDataSource());
      Account account = null;
      try {
          account = queryRunner.query("select * from account where id = ?", new ResultSetHandler<Account>(){
              @Override
              public Account handle(ResultSet rs) throws SQLException {
                  Account account = new Account();
                  while(rs.next()){
                      String name = rs.getString("name");
                      int money = rs.getInt("money");
                      account.setName(name);
                      account.setMoney(money);
                  }
                  return account;
              }
          }, 3);
      } catch (SQLException e) {
          e.printStackTrace();
      }
      System.out.println(account.toString());
  }

直接使用框架已经写好的实现类

# 查询单个对象
QueryRunner queryRunner = new QueryRunner(new ComboPooledDataSource());
Account account = queryRunner.query("select * from account where id = ?",
new BeanHandler<Account>(Account.class), 8);
# 查询多个对象
QueryRunner queryRunner = new QueryRunner(new ComboPooledDataSource());
List<Account> list = queryRunner.query("select * from account ",
new BeanListHandler<Account>(Account.class));

ResultSetHandler 常用的实现类

BeanHandler：查询到的单个数据封装成一个对象

BeanListHandler：查询到的多个数据封装成一个List<对象>

ArrayHandler：查询到的单个数据封装成一个数组

ArrayListHandler,：查询到的多个数据封装成一个集合，集合里面的元素是数组

MapListHandler：查询到的多个数据封装成一个集合，集合里面的元素是map

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