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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