Mybaits 源码剖析 （七）----- Select 语句的实行历程剖析（下篇）
2019-11-18杂谈搜奇网45°c
A+ A-我们上篇文章讲到了查询要领内里的doQuery要领，这内里就是挪用JDBC的API了，个中的逻辑比较复杂，我们这边文章来讲，先看看我们上篇文章剖析的处所
SimpleExecutor
1 public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException { 2 Statement stmt = null; 3 try { 4 Configuration configuration = ms.getConfiguration(); 5 // 建立 StatementHandler 6 StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql); 7 // 建立 Statement 8 stmt = prepareStatement(handler, ms.getStatementLog()); 9 // 实行查询操纵 10 return handler.<E>query(stmt, resultHandler); 11 } finally { 12 // 封闭 Statement 13 closeStatement(stmt); 14 } 15 }
上篇文章我们剖析完了第6行代码，在第6行处我们建立了一个PreparedStatementHandler，我们要接着第8行代码最先剖析，也就是建立 Statement，先不忙着剖析，我们先来回忆一下 ，我们之前是怎样运用jdbc的
jdbc
public class Login { /** * 第一步，加载驱动，建立数据库的衔接 * 第二步，编写sql * 第三步，须要对sql举行预编译 * 第四步，向sql内里设置参数 * 第五步，实行sql * 第六步，开释资本 * @throws Exception */ public static final String URL = "jdbc:mysql://localhost:3306/chenhao"; public static final String USER = "liulx"; public static final String PASSWORD = "123456"; public static void main(String[] args) throws Exception { login("lucy","123"); } public static void login(String username , String password) throws Exception{ Connection conn = null; PreparedStatement psmt = null; ResultSet rs = null; try { //加载驱动顺序 Class.forName("com.mysql.jdbc.Driver"); //获得数据库衔接 conn = DriverManager.getConnection(URL, USER, PASSWORD); //编写sql String sql = "select * from user where name =? and password = ?";//问号相当于一个占位符 //对sql举行预编译 psmt = conn.prepareStatement(sql); //设置参数 psmt.setString(1, username); psmt.setString(2, password); //实行sql ,返回一个效果集 rs = psmt.executeQuery(); //输出效果 while(rs.next()){ System.out.println(rs.getString("user_name")+" 岁数："+rs.getInt("age")); } } catch (Exception e) { e.printStackTrace(); }finally{ //开释资本 conn.close(); psmt.close(); rs.close(); } } }
上面代码中解释已很清晰了，我们来看看mybatis中是怎样和数据库打交道的。
SimpleExecutor
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException { Statement stmt; // 猎取数据库衔接 Connection connection = getConnection(statementLog); // 建立 Statement， stmt = handler.prepare(connection, transaction.getTimeout()); // 为 Statement 设置参数 handler.parameterize(stmt); return stmt; }
在上面的代码中我们终究看到了和jdbc相干的内容了，也许分为下面三个步骤：
- 猎取数据库衔接
- 建立PreparedStatement
- 为PreparedStatement设置运转时参数
我们先来看看猎取数据库衔接，跟进代码看看
BaseExecutor
protected Connection getConnection(Log statementLog) throws SQLException { //经由历程transaction来猎取Connection Connection connection = this.transaction.getConnection(); return statementLog.isDebugEnabled() ? ConnectionLogger.newInstance(connection, statementLog, this.queryStack) : connection; }
我们看到是经由历程Executor中的transaction属性来猎取Connection，那我们就先来看看transaction，依据前面的文章中的设置 <transactionManager type="jdbc"/>，则MyBatis会建立一个JdbcTransactionFactory.class 实例，Executor中的transaction是一个JdbcTransaction.class 实例，其完成Transaction接口，那我们先来看看Transaction
JdbcTransaction
我们先来看看其接口Transaction
Transaction
public interface Transaction { //猎取数据库衔接 Connection getConnection() throws SQLException; //提交事件 void commit() throws SQLException; //回滚事件 void rollback() throws SQLException; //封闭事件 void close() throws SQLException; //猎取超时时刻 Integer getTimeout() throws SQLException; }
接着我们看看其完成类JdbcTransaction
JdbcTransaction
public class JdbcTransaction implements Transaction { private static final Log log = LogFactory.getLog(JdbcTransaction.class); //数据库衔接 protected Connection connection; //数据源信息 protected DataSource dataSource; //断绝级别 protected TransactionIsolationLevel level; //是不是为自动提交 protected boolean autoCommmit; public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) { dataSource = ds; level = desiredLevel; autoCommmit = desiredAutoCommit; } public JdbcTransaction(Connection connection) { this.connection = connection; } public Connection getConnection() throws SQLException { //假如事件中不存在connection，则猎取一个connection并放入connection属性中 //第一次肯定为空 if (connection == null) { openConnection(); } //假如事件中已存在connection，则直接返回这个connection return connection; } /** * commit()功用 * @throws SQLException */ public void commit() throws SQLException { if (connection != null && !connection.getAutoCommit()) { if (log.isDebugEnabled()) { log.debug("Committing JDBC Connection [" + connection + "]"); } //运用connection的commit() connection.commit(); } } /** * rollback()功用 * @throws SQLException */ public void rollback() throws SQLException { if (connection != null && !connection.getAutoCommit()) { if (log.isDebugEnabled()) { log.debug("Rolling back JDBC Connection [" + connection + "]"); } //运用connection的rollback() connection.rollback(); } } /** * close()功用 * @throws SQLException */ public void close() throws SQLException { if (connection != null) { resetAutoCommit(); if (log.isDebugEnabled()) { log.debug("Closing JDBC Connection [" + connection + "]"); } //运用connection的close() connection.close(); } } protected void openConnection() throws SQLException { if (log.isDebugEnabled()) { log.debug("Opening JDBC Connection"); } //经由历程dataSource来猎取connection，并设置到transaction的connection属性中 connection = dataSource.getConnection(); if (level != null) { //经由历程connection设置事件的断绝级别 connection.setTransactionIsolation(level.getLevel()); } //设置事件是不是自动提交 setDesiredAutoCommit(autoCommmit); } protected void setDesiredAutoCommit(boolean desiredAutoCommit) { try { if (this.connection.getAutoCommit() != desiredAutoCommit) { if (log.isDebugEnabled()) { log.debug("Setting autocommit to " + desiredAutoCommit + " on JDBC Connection [" + this.connection + "]"); } //经由历程connection设置事件是不是自动提交 this.connection.setAutoCommit(desiredAutoCommit); } } catch (SQLException var3) { throw new TransactionException("Error configuring AutoCommit. Your driver may not support getAutoCommit() or setAutoCommit(). Requested setting: " + desiredAutoCommit + ". Cause: " + var3, var3); } } }
我们看到JdbcTransaction中有一个Connection属性和dataSource属性，运用connection来举行提交、回滚、封闭等操纵，也就是说JdbcTransaction实在只是在jdbc的connection上面封装了一下，现实运用的实在照样jdbc的事件。我们看看getConnection()要领
//数据库衔接 protected Connection connection; //数据源信息 protected DataSource dataSource; public Connection getConnection() throws SQLException { //假如事件中不存在connection，则猎取一个connection并放入connection属性中 //第一次肯定为空 if (connection == null) { openConnection(); } //假如事件中已存在connection，则直接返回这个connection return connection; } protected void openConnection() throws SQLException { if (log.isDebugEnabled()) { log.debug("Opening JDBC Connection"); } //经由历程dataSource来猎取connection，并设置到transaction的connection属性中 connection = dataSource.getConnection(); if (level != null) { //经由历程connection设置事件的断绝级别 connection.setTransactionIsolation(level.getLevel()); } //设置事件是不是自动提交 setDesiredAutoCommit(autoCommmit); }
先是推断当前事件中是不是存在connection，假如存在，则直接返回connection，假如不存在则经由历程dataSource来猎取connection，这里我们邃晓了一点，假如当前事件没有封闭，也就是没有开释connection，那末在同一个Transaction中运用的是同一个connection,我们再来想一想，transaction是SimpleExecutor中的属性，SimpleExecutor又是SqlSession中的属性，那我们能够如许说，同一个SqlSession中只需一个SimpleExecutor，SimpleExecutor中有一个Transaction，Transaction有一个connection。我们来看看以下例子
public static void main(String[] args) throws IOException { String resource = "mybatis-config.xml"; InputStream inputStream = Resources.getResourceAsStream(resource); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); //建立一个SqlSession SqlSession sqlSession = sqlSessionFactory.openSession(); try { EmployeeMapper employeeMapper = sqlSession.getMapper(Employee.class); UserMapper userMapper = sqlSession.getMapper(User.class); List<Employee> allEmployee = employeeMapper.getAll(); List<User> allUser = userMapper.getAll(); Employee employee = employeeMapper.getOne(); } finally { sqlSession.close(); } }
我们看到同一个sqlSession能够猎取多个Mapper代办对象，则多个Mapper代办对象中的sqlSession援用应该是同一个，那末多个Mapper代办对象挪用要领应该是同一个Connection，直到挪用close(),所以说我们的sqlSession是线程不安全的，假如一切的营业都运用一个sqlSession，那Connection也是同一个，一个营业实行完了就将其封闭，那其他的营业还没实行完呢。人人邃晓了吗？我们回归到源码，connection = dataSource.getConnection();，终究照样挪用dataSource来猎取衔接，那我们是不是是要来看看dataSource呢？
我们照样夙昔面的设置文件来看<dataSource type="UNPOOLED|POOLED">，这里有UNPOOLED和POOLED两种DataSource，一种是运用衔接池，一种是平常的DataSource，UNPOOLED将会创将new UnpooledDataSource()实例，POOLED将会new pooledDataSource()实例，都完成DataSource接口，那我们先来看看DataSource接口
DataSource
public interface DataSource extends CommonDataSource,Wrapper { //猎取数据库衔接 Connection getConnection() throws SQLException; Connection getConnection(String username, String password) throws SQLException; }
很简朴，只需一个猎取数据库衔接的接口，那我们来看看其完成类
UnpooledDataSource
UnpooledDataSource，从称号上即可晓得，该种数据源不具有池化特征。该种数据源每次会返回一个新的数据库衔接，而非复用旧的衔接。其中心的要领有三个，离别以下：
- initializeDriver - 初始化数据库驱动
- doGetConnection - 猎取数据衔接
- configureConnection - 设置数据库衔接
初始化数据库驱动
看下我们上面运用JDBC的例子，在实行 SQL 之前，平常都是先猎取数据库衔接。平常步骤都是加载数据库驱动，然后经由历程 DriverManager 猎取数据库衔接。UnpooledDataSource 也是运用 JDBC 接见数据库的，因而它猎取数据库衔接的历程一样
UnpooledDataSource
public class UnpooledDataSource implements DataSource { private ClassLoader driverClassLoader; private Properties driverProperties; private static Map<String, Driver> registeredDrivers = new ConcurrentHashMap(); private String driver; private String url; private String username; private String password; private Boolean autoCommit; private Integer defaultTransactionIsolationLevel; public UnpooledDataSource() { } public UnpooledDataSource(String driver, String url, String username, String password) { this.driver = driver; this.url = url; this.username = username; this.password = password; } private synchronized void initializeDriver() throws SQLException { // 检测当前 driver 对应的驱动实例是不是已注册 if (!registeredDrivers.containsKey(driver)) { Class<?> driverType; try { // 加载驱动范例 if (driverClassLoader != null) { // 运用 driverClassLoader 加载驱动 driverType = Class.forName(driver, true, driverClassLoader); } else { // 经由历程其他 ClassLoader 加载驱动 driverType = Resources.classForName(driver); } // 经由历程反射建立驱动实例 Driver driverInstance = (Driver) driverType.newInstance(); /* * 注册驱动，注重这里是将 Driver 代办类 DriverProxy 对象注册到 DriverManager 中的，而非 Driver 对象本身。 */ DriverManager.registerDriver(new DriverProxy(driverInstance)); // 缓存驱动类名和实例，防备屡次注册 registeredDrivers.put(driver, driverInstance); } catch (Exception e) { throw new SQLException("Error setting driver on UnpooledDataSource. Cause: " + e); } } } //略... } //DriverManager private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<DriverInfo>(); public static synchronized void registerDriver(java.sql.Driver driver) throws SQLException { if(driver != null) { registeredDrivers.addIfAbsent(new DriverInfo(driver)); } else { // This is for compatibility with the original DriverManager throw new NullPointerException(); } }
经由历程反射机制加载驱动Driver，并将其注册到DriverManager中的一个常量鸠合中，供背面猎取衔接时运用，为何这里是一个List呢？我们现实开辟中有能够运用到了多种数据库范例，如Mysql、Oracle等，其驱动都是差别的，差别的数据源猎取衔接时运用的是差别的驱动。
在我们运用JDBC的时刻，也没有经由历程DriverManager.registerDriver(new DriverProxy(driverInstance));去注册Driver啊，假如我们运用的是Mysql数据源，那我们来看Class.forName("com.mysql.jdbc.Driver");这句代码发生了什么
Class.forName主如果做了什么呢？它主如果要求JVM查找并装载指定的类。如许我们的类com.mysql.jdbc.Driver就被装载进来了。而且在类被装载进JVM的时刻，它的静态要领就会被实行。我们来看com.mysql.jdbc.Driver的完成代码。在它的完成里有这么一段代码：
static { try { java.sql.DriverManager.registerDriver(new Driver()); } catch (SQLException E) { throw new RuntimeException("Can't register driver!"); } }
很明显，这里运用了DriverManager并将该类给注册上去了。所以，关于任何完成前面Driver接口的类，只需在他们被装载进JVM的时刻注册DriverManager就能够完成被后续顺序运用。
作为那些被加载的Driver完成，他们本身在被装载时会在实行的static代码段里经由历程挪用DriverManager.registerDriver()来把本身注册到DriverManager的registeredDrivers列表中。如许背面就能够经由历程获得的Driver来获得衔接了。
猎取数据库衔接
在上面例子中运用 JDBC 时，我们都是经由历程 DriverManager 的接口要领猎取数据库衔接。我们来看看UnpooledDataSource是怎样猎取的。
UnpooledDataSource
public Connection getConnection() throws SQLException { return doGetConnection(username, password); } private Connection doGetConnection(String username, String password) throws SQLException { Properties props = new Properties(); if (driverProperties != null) { props.putAll(driverProperties); } if (username != null) { // 存储 user 设置 props.setProperty("user", username); } if (password != null) { // 存储 password 设置 props.setProperty("password", password); } // 挪用重载要领 return doGetConnection(props); } private Connection doGetConnection(Properties properties) throws SQLException { // 初始化驱动,我们上一节已讲过了，只用初始化一次 initializeDriver(); // 猎取衔接 Connection connection = DriverManager.getConnection(url, properties); // 设置衔接，包含自动提交以及事件品级 configureConnection(connection); return connection; } private void configureConnection(Connection conn) throws SQLException { if (autoCommit != null && autoCommit != conn.getAutoCommit()) { // 设置自动提交 conn.setAutoCommit(autoCommit); } if (defaultTransactionIsolationLevel != null) { // 设置事件断绝级别 conn.setTransactionIsolation(defaultTransactionIsolationLevel); } }
上面要领将一些设置信息放入到 Properties 对象中，然后将数据库衔接和 Properties 对象传给 DriverManager 的 getConnection 要领即可猎取到数据库衔接。我们来看看是怎样猎取数据库衔接的
private static Connection getConnection(String url, java.util.Properties info, Class<?> caller) throws SQLException { // 猎取类加载器 ClassLoader callerCL = caller != null ? caller.getClassLoader() : null; synchronized(DriverManager.class) { if (callerCL == null) { callerCL = Thread.currentThread().getContextClassLoader(); } } // 此处省略部份代码 // 这里遍历的是在registerDriver(Driver driver)要领中注册的驱动对象 // 每一个DriverInfo包含了驱动对象和其信息 for(DriverInfo aDriver : registeredDrivers) { // 推断是不是为当前线程类加载器加载的驱动类 if(isDriverAllowed(aDriver.driver, callerCL)) { try { println("trying " + aDriver.driver.getClass().getName()); // 猎取衔接对象，这里挪用了Driver的父类的要领 // 假如这里有多个DriverInfo，比方Mysql和Oracle的Driver都注册registeredDrivers了 // 这里一切的Driver都邑尝试运用url和info去衔接，哪一个衔接上了就返回 // 会不会一切的都邑衔接上呢？不会，因为url的写法差别，差别的Driver会推断url是不是合适当前驱动 Connection con = aDriver.driver.connect(url, info); if (con != null) { // 打印衔接胜利信息 println("getConnection returning " + aDriver.driver.getClass().getName()); // 返回衔接对像 return (con); } } catch (SQLException ex) { if (reason == null) { reason = ex; } } } else { println(" skipping: " + aDriver.getClass().getName()); } } }
代码中轮回一切注册的驱动，然后经由历程驱动举行衔接，一切的驱动都邑尝试衔接，然则差别的驱动，衔接的URL是差别的，如Mysql的url是jdbc:mysql://localhost:3306/chenhao，以jdbc:mysql://开首，则其Mysql的驱动肯定会推断猎取衔接的url相符，Oracle的也相似，我们来看看Mysql的驱动猎取衔接
因为篇幅缘由，我这里就不剖析了，人人有兴致的能够看看，末了由URL对应的驱动猎取到Connection返回，好了我们再来看看下一种DataSource
PooledDataSource
PooledDataSource 内部完成了衔接池功用，用于复用数据库衔接。因而，从效力上来讲，PooledDataSource 要高于 UnpooledDataSource。然则终究猎取Connection照样经由历程UnpooledDataSource，只不过PooledDataSource 供应一个存储Connection的功用。
辅佐类引见
PooledDataSource 须要借助两个辅佐类帮其完胜利用，这两个辅佐类离别是 PoolState 和 PooledConnection。PoolState 用于纪录衔接池运转时的状况，比方衔接猎取次数，无效衔接数目等。同时 PoolState 内部定义了两个 PooledConnection 鸠合，用于存储余暇衔接和活泼衔接。PooledConnection 内部定义了一个 Connection 范例的变量，用于指向实在的数据库衔接。以及一个 Connection 的代办类，用于对部份要领挪用举行阻拦。至于为何要阻拦，随后将举行剖析。除此之外，PooledConnection 内部也定义了一些字段，用于纪录数据库衔接的一些运转时状况。接下来，我们来看一下 PooledConnection 的定义。
PooledConnection
class PooledConnection implements InvocationHandler { private static final String CLOSE = "close"; private static final Class<?>[] IFACES = new Class<?>[]{Connection.class}; private final int hashCode; private final PooledDataSource dataSource; // 实在的数据库衔接 private final Connection realConnection; // 数据库衔接代办 private final Connection proxyConnection; // 从衔接池中掏出衔接时的时刻戳 private long checkoutTimestamp; // 数据库衔接建立时刻 private long createdTimestamp; // 数据库衔接末了运用时刻 private long lastUsedTimestamp; // connectionTypeCode = (url + username + password).hashCode() private int connectionTypeCode; // 示意衔接是不是有用 private boolean valid; public PooledConnection(Connection connection, PooledDataSource dataSource) { this.hashCode = connection.hashCode(); this.realConnection = connection; this.dataSource = dataSource; this.createdTimestamp = System.currentTimeMillis(); this.lastUsedTimestamp = System.currentTimeMillis(); this.valid = true; // 建立 Connection 的代办类对象 this.proxyConnection = (Connection) Proxy.newProxyInstance(Connection.class.getClassLoader(), IFACES, this); } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {...} // 省略部份代码 }
下面再来看看 PoolState 的定义。
PoolState
public class PoolState { protected PooledDataSource dataSource; // 余暇衔接列表 protected final List<PooledConnection> idleConnections = new ArrayList<PooledConnection>(); // 活泼衔接列表 protected final List<PooledConnection> activeConnections = new ArrayList<PooledConnection>(); // 从衔接池中猎取衔接的次数 protected long requestCount = 0; // 要求衔接总耗时（单元：毫秒） protected long accumulatedRequestTime = 0; // 衔接实行时刻总耗时 protected long accumulatedCheckoutTime = 0; // 实行时刻超时的衔接数 protected long claimedOverdueConnectionCount = 0; // 超时时刻累加值 protected long accumulatedCheckoutTimeOfOverdueConnections = 0; // 守候时刻累加值 protected long accumulatedWaitTime = 0; // 守候次数 protected long hadToWaitCount = 0; // 无效衔接数 protected long badConnectionCount = 0; }
人人记着上面的余暇衔接列表和活泼衔接列表
猎取衔接
前面已说过，PooledDataSource 会将用过的衔接举行接纳，以便能够复用衔接。因而从 PooledDataSource 猎取衔接时，假如余暇链接列内外有衔接时，可直接取用。那假如没有余暇衔接怎样办呢？此时有两种解决办法，要么建立新衔接，要么守候其他衔接完成任务。
PooledDataSource
public class PooledDataSource implements DataSource { private static final Log log = LogFactory.getLog(PooledDataSource.class); //这里有辅佐类PoolState private final PoolState state = new PoolState(this); //另有一个UnpooledDataSource属性，实在真正猎取Connection是由UnpooledDataSource来完成的 private final UnpooledDataSource dataSource; protected int poolMaximumActiveConnections = 10; protected int poolMaximumIdleConnections = 5; protected int poolMaximumCheckoutTime = 20000; protected int poolTimeToWait = 20000; protected String poolPingQuery = "NO PING QUERY SET"; protected boolean poolPingEnabled = false; protected int poolPingConnectionsNotUsedFor = 0; private int expectedConnectionTypeCode; public PooledDataSource() { this.dataSource = new UnpooledDataSource(); } public PooledDataSource(String driver, String url, String username, String password) { //组织器中建立UnpooledDataSource对象 this.dataSource = new UnpooledDataSource(driver, url, username, password); } public Connection getConnection() throws SQLException { return this.popConnection(this.dataSource.getUsername(), this.dataSource.getPassword()).getProxyConnection(); } private PooledConnection popConnection(String username, String password) throws SQLException { boolean countedWait = false; PooledConnection conn = null; long t = System.currentTimeMillis(); int localBadConnectionCount = 0; while (conn == null) { synchronized (state) { // 检测余暇衔接鸠合（idleConnections）是不是为空 if (!state.idleConnections.isEmpty()) { // idleConnections 不为空，示意有余暇衔接能够运用，直接从余暇衔接鸠合中掏出一个衔接 conn = state.idleConnections.remove(0); } else { /* * 暂无余暇衔接可用，但假如活泼衔接数还未超越限定 *（poolMaximumActiveConnections），则可建立新的衔接 */ if (state.activeConnections.size() < poolMaximumActiveConnections) { // 建立新衔接，看到没，照样经由历程dataSource猎取衔接，也就是UnpooledDataSource猎取衔接 conn = new PooledConnection(dataSource.getConnection(), this); } else { // 衔接池已满，不能建立新衔接 // 掏出运转时刻最长的衔接 PooledConnection oldestActiveConnection = state.activeConnections.get(0); // 猎取运转时长 long longestCheckoutTime = oldestActiveConnection.getCheckoutTime(); // 检测运转时长是不是超越限定，即超时 if (longestCheckoutTime > poolMaximumCheckoutTime) { // 累加超时相干的统计字段 state.claimedOverdueConnectionCount++; state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime; state.accumulatedCheckoutTime += longestCheckoutTime; // 从活泼衔接鸠合中移除超时衔接 state.activeConnections.remove(oldestActiveConnection); // 若衔接未设置自动提交，此处举行回滚操纵 if (!oldestActiveConnection.getRealConnection().getAutoCommit()) { try { oldestActiveConnection.getRealConnection().rollback(); } catch (SQLException e) {...} } /* * 建立一个新的 PooledConnection，注重， * 此处复用 oldestActiveConnection 的 realConnection 变量 */ conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this); /* * 复用 oldestActiveConnection 的一些信息，注重 PooledConnection 中的 * createdTimestamp 用于纪录 Connection 的建立时刻，而非 PooledConnection * 的建立时刻。所以这里要复用原衔接的时刻信息。 */ conn.setCreatedTimestamp(oldestActiveConnection.getCreatedTimestamp()); conn.setLastUsedTimestamp(oldestActiveConnection.getLastUsedTimestamp()); // 设置衔接为无效状况 oldestActiveConnection.invalidate(); } else {// 运转时刻最长的衔接并未超时 try { if (!countedWait) { state.hadToWaitCount++; countedWait = true; } long wt = System.currentTimeMillis(); // 当前线程进入守候状况 state.wait(poolTimeToWait); state.accumulatedWaitTime += System.currentTimeMillis() - wt; } catch (InterruptedException e) { break; } } } } if (conn != null) { if (conn.isValid()) { if (!conn.getRealConnection().getAutoCommit()) { // 举行回滚操纵 conn.getRealConnection().rollback(); } conn.setConnectionTypeCode(assembleConnectionTypeCode(dataSource.getUrl(), username, password)); // 设置统计字段 conn.setCheckoutTimestamp(System.currentTimeMillis()); conn.setLastUsedTimestamp(System.currentTimeMillis()); state.activeConnections.add(conn); state.requestCount++; state.accumulatedRequestTime += System.currentTimeMillis() - t; } else { // 衔接无效，此时累加无效衔接相干的统计字段 state.badConnectionCount++; localBadConnectionCount++; conn = null; if (localBadConnectionCount > (poolMaximumIdleConnections + poolMaximumLocalBadConnectionTolerance)) { throw new SQLException(...); } } } } } if (conn == null) { throw new SQLException(...); } return conn; } }
从衔接池中猎取衔接起首会碰到两种状况：
- 衔接池中有余暇衔接
- 衔接池中无余暇衔接
关于第一种状况，把衔接掏出返回即可。关于第二种状况，则要举行细分，会有以下的状况。
- 活泼衔接数没有超越最大活泼衔接数
- 活泼衔接数超越最大活泼衔接数
关于上面两种状况，第一种状况比较好处置惩罚，直接建立新的衔接即可。至于第二种状况，须要再次举行细分。
- 活泼衔接的运转时刻超越限定，即超时了
- 活泼衔接未超时
关于第一种状况，我们直接将超时衔接强行中断，并举行回滚，然后复用部份字段从新建立 PooledConnection 即可。关于第二种状况，如今没有更好的处置惩罚方式了，只能守候了。
接纳衔接
比拟于猎取衔接，接纳衔接的逻辑要简朴的多。接纳衔接胜利与否只取决于余暇衔接鸠合的状况，所需处置惩罚状况很少，因而比较简朴。
我们照样来看看
public Connection getConnection() throws SQLException { return this.popConnection(this.dataSource.getUsername(), this.dataSource.getPassword()).getProxyConnection(); }
返回的是PooledConnection的一个代办类，为何不直接运用PooledConnection的realConnection呢？我们能够看下PooledConnection这个类
class PooledConnection implements InvocationHandler {
很熟悉是吧，规范的代办类用法，看下其invoke要领
PooledConnection
@Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { String methodName = method.getName(); // 重点在这里，假如挪用了其close要领，则现实实行的是将衔接放回衔接池的操纵 if (CLOSE.hashCode() == methodName.hashCode() && CLOSE.equals(methodName)) { dataSource.pushConnection(this); return null; } else { try { if (!Object.class.equals(method.getDeclaringClass())) { // issue #579 toString() should never fail // throw an SQLException instead of a Runtime checkConnection(); } // 其他的操纵都交给realConnection实行 return method.invoke(realConnection, args); } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } } }
那我们来看看pushConnection做了什么
protected void pushConnection(PooledConnection conn) throws SQLException { synchronized (state) { // 从活泼衔接池中移除衔接 state.activeConnections.remove(conn); if (conn.isValid()) { // 余暇衔接鸠合未满 if (state.idleConnections.size() < poolMaximumIdleConnections && conn.getConnectionTypeCode() == expectedConnectionTypeCode) { state.accumulatedCheckoutTime += conn.getCheckoutTime(); // 回滚未提交的事件 if (!conn.getRealConnection().getAutoCommit()) { conn.getRealConnection().rollback(); } // 建立新的 PooledConnection PooledConnection newConn = new PooledConnection(conn.getRealConnection(), this); state.idleConnections.add(newConn); // 复用时刻信息 newConn.setCreatedTimestamp(conn.getCreatedTimestamp()); newConn.setLastUsedTimestamp(conn.getLastUsedTimestamp()); // 将原衔接置为无效状况 conn.invalidate(); // 关照守候的线程 state.notifyAll(); } else {// 余暇衔接鸠合已满 state.accumulatedCheckoutTime += conn.getCheckoutTime(); // 回滚未提交的事件 if (!conn.getRealConnection().getAutoCommit()) { conn.getRealConnection().rollback(); } // 封闭数据库衔接 conn.getRealConnection().close(); conn.invalidate(); } } else { state.badConnectionCount++; } } }
先将衔接从活泼衔接鸠合中移除，假如余暇鸠合未满，此时复用原衔接的字段信息建立新的衔接，并将其放入余暇鸠合中即可；若余暇鸠合已满，此时无需接纳衔接，直接封闭即可。
衔接池总觉得很神奇，但仔细剖析完其代码以后，也就没那末神奇了，就是将衔接运用完以后放到一个鸠合中，下面再猎取衔接的时刻起首从这个鸠合中猎取。 另有PooledConnection的代办情势的运用，值得我们进修
好了，我们已猎取到了数据库衔接，接下来要建立PrepareStatement了，我们上面JDBC的例子是怎样猎取的？ psmt = conn.prepareStatement(sql);，直接经由历程Connection来猎取，而且把sql传进去了，我们看看Mybaits中是怎样建立PrepareStatement的
建立PreparedStatement
PreparedStatementHandler
stmt = handler.prepare(connection, transaction.getTimeout()); public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException { Statement statement = null; try { // 建立 Statement statement = instantiateStatement(connection); // 设置超时和 FetchSize setStatementTimeout(statement, transactionTimeout); setFetchSize(statement); return statement; } catch (SQLException e) { closeStatement(statement); throw e; } catch (Exception e) { closeStatement(statement); throw new ExecutorException("Error preparing statement. Cause: " + e, e); } } protected Statement instantiateStatement(Connection connection) throws SQLException { //猎取sql字符串，比方"select * from user where id= ?" String sql = boundSql.getSql(); // 依据前提挪用差别的 prepareStatement 要领建立 PreparedStatement if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) { String[] keyColumnNames = mappedStatement.getKeyColumns(); if (keyColumnNames == null) { //经由历程connection猎取Statement，将sql语句传进去 return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS); } else { return connection.prepareStatement(sql, keyColumnNames); } } else if (mappedStatement.getResultSetType() != null) { return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY); } else { return connection.prepareStatement(sql); } }
看到没和jdbc的情势如出一辙，我们详细来看看connection.prepareStatement做了什么
1 public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency) throws SQLException { 2 3 boolean canServerPrepare = true; 4 5 String nativeSql = getProcessEscapeCodesForPrepStmts() ? nativeSQL(sql) : sql; 6 7 if (this.useServerPreparedStmts && getEmulateUnsupportedPstmts()) { 8 canServerPrepare = canHandleAsServerPreparedStatement(nativeSql); 9 } 10 11 if (this.useServerPreparedStmts && getEmulateUnsupportedPstmts()) { 12 canServerPrepare = canHandleAsServerPreparedStatement(nativeSql); 13 } 14 15 if (this.useServerPreparedStmts && canServerPrepare) { 16 if (this.getCachePreparedStatements()) { 17 ...... 18 } else { 19 try { 20 //这里运用的是ServerPreparedStatement建立PreparedStatement 21 pStmt = ServerPreparedStatement.getInstance(getMultiHostSafeProxy(), nativeSql, this.database, resultSetType, resultSetConcurrency); 22 23 pStmt.setResultSetType(resultSetType); 24 pStmt.setResultSetConcurrency(resultSetConcurrency); 25 } catch (SQLException sqlEx) { 26 // Punt, if necessary 27 if (getEmulateUnsupportedPstmts()) { 28 pStmt = (PreparedStatement) clientPrepareStatement(nativeSql, resultSetType, resultSetConcurrency, false); 29 } else { 30 throw sqlEx; 31 } 32 } 33 } 34 } else { 35 pStmt = (PreparedStatement) clientPrepareStatement(nativeSql, resultSetType, resultSetConcurrency, false); 36 } 37 }
我们只用看最症结的第21行代码，运用ServerPreparedStatement的getInstance返回一个PreparedStatement，实在本质上ServerPreparedStatement继承了PreparedStatement对象，我们看看其组织要领
protected ServerPreparedStatement(ConnectionImpl conn, String sql, String catalog, int resultSetType, int resultSetConcurrency) throws SQLException { //略... try { this.serverPrepare(sql); } catch (SQLException var10) { this.realClose(false, true); throw var10; } catch (Exception var11) { this.realClose(false, true); SQLException sqlEx = SQLError.createSQLException(var11.toString(), "S1000", this.getExceptionInterceptor()); sqlEx.initCause(var11); throw sqlEx; } //略... }
继承挪用this.serverPrepare(sql);
public class ServerPreparedStatement extends PreparedStatement { //寄存运转时参数的数组 private ServerPreparedStatement.BindValue[] parameterBindings; //服务器预编译好的sql语句返回的serverStatementId private long serverStatementId; private void serverPrepare(String sql) throws SQLException { synchronized(this.connection.getMutex()) { MysqlIO mysql = this.connection.getIO(); try { //向sql服务器发送了一条PREPARE指令 Buffer prepareResultPacket = mysql.sendCommand(MysqlDefs.COM_PREPARE, sql, (Buffer)null, false, characterEncoding, 0); //纪录下了预编译好的sql语句所对应的serverStatementId this.serverStatementId = prepareResultPacket.readLong(); this.fieldCount = prepareResultPacket.readInt(); //猎取参数个数，比方 select * from user where id= ？and name = ？，个中有两个？，则这里返回的参数个数应该为2 this.parameterCount = prepareResultPacket.readInt(); this.parameterBindings = new ServerPreparedStatement.BindValue[this.parameterCount]; for(int i = 0; i < this.parameterCount; ++i) { //依据参数个数，初始化数组 this.parameterBindings[i] = new ServerPreparedStatement.BindValue(); } } catch (SQLException var16) { throw sqlEx; } finally { this.connection.getIO().clearInputStream(); } } } }
ServerPreparedStatement继承PreparedStatement，ServerPreparedStatement初始化的时刻就向sql服务器发送了一条PREPARE指令，把SQL语句传到mysql服务器，如select * from user where id= ？and name = ？，mysql服务器会对sql举行编译，并保存在服务器，返回预编译语句对应的id，并保存在
ServerPreparedStatement中，同时建立BindValue[] parameterBindings数组，背面设置参数就直接增加到此数组中。好了，此时我们建立了一个ServerPreparedStatement并返回，下面就是设置运转时参数了
设置运转时参数到 SQL 中
我们已猎取到了PreparedStatement，接下来就是将运转时参数设置到PreparedStatement中，以下代码
handler.parameterize(stmt);
JDBC是怎样设置的呢？我们看看上面的例子，很简朴吧
psmt = conn.prepareStatement(sql); //设置参数 psmt.setString(1, username); psmt.setString(2, password);
我们来看看parameterize要领
public void parameterize(Statement statement) throws SQLException { // 经由历程参数处置惩罚器 ParameterHandler 设置运转时参数到 PreparedStatement 中 parameterHandler.setParameters((PreparedStatement) statement); } public class DefaultParameterHandler implements ParameterHandler { private final TypeHandlerRegistry typeHandlerRegistry; private final MappedStatement mappedStatement; private final Object parameterObject; private final BoundSql boundSql; private final Configuration configuration; public void setParameters(PreparedStatement ps) { /* * 从 BoundSql 中猎取 ParameterMapping 列表，每一个 ParameterMapping 与原始 SQL 中的 #{xxx} 占位符一一对应 */ List<ParameterMapping> parameterMappings = boundSql.getParameterMappings(); if (parameterMappings != null) { for (int i = 0; i < parameterMappings.size(); i++) { ParameterMapping parameterMapping = parameterMappings.get(i); if (parameterMapping.getMode() != ParameterMode.OUT) { Object value; // 猎取属性名 String propertyName = parameterMapping.getProperty(); if (boundSql.hasAdditionalParameter(propertyName)) { value = boundSql.getAdditionalParameter(propertyName); } else if (parameterObject == null) { value = null; } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) { value = parameterObject; } else { // 为用户传入的参数 parameterObject 建立元信息对象 MetaObject metaObject = configuration.newMetaObject(parameterObject); // 从用户传入的参数中猎取 propertyName 对应的值 value = metaObject.getValue(propertyName); } TypeHandler typeHandler = parameterMapping.getTypeHandler(); JdbcType jdbcType = parameterMapping.getJdbcType(); if (value == null && jdbcType == null) { jdbcType = configuration.getJdbcTypeForNull(); } try { // 由范例处置惩罚器 typeHandler 向 ParameterHandler 设置参数 typeHandler.setParameter(ps, i + 1, value, jdbcType); } catch (TypeException e) { throw new TypeException(...); } catch (SQLException e) { throw new TypeException(...); } } } } } }
起首从boundSql中猎取parameterMappings 鸠合，这块人人能够看看我前面的文章，然后遍历猎取 parameterMapping中的propertyName ，如#{name} 中的name,然后从运转时参数parameterObject中猎取name对应的参数值，末了设置到PreparedStatement 中，我们重要来看是怎样设置参数的。也就是
typeHandler.setParameter(ps, i + 1, value, jdbcType);，这句代码终究会向我们例子中一样实行，以下
public void setNonNullParameter(PreparedStatement ps, int i, String parameter, JdbcType jdbcType) throws SQLException { ps.setString(i, parameter); }
还记得我们的PreparedStatement是什么吗？是ServerPreparedStatement，那我们就来看看ServerPreparedStatement的setString要领
public void setString(int parameterIndex, String x) throws SQLException { this.checkClosed(); if (x == null) { this.setNull(parameterIndex, 1); } else { //依据参数下标从parameterBindings数组总猎取BindValue ServerPreparedStatement.BindValue binding = this.getBinding(parameterIndex, false); this.setType(binding, this.stringTypeCode); //设置参数值 binding.value = x; binding.isNull = false; binding.isLongData = false; } } protected ServerPreparedStatement.BindValue getBinding(int parameterIndex, boolean forLongData) throws SQLException { this.checkClosed(); if (this.parameterBindings.length == 0) { throw SQLError.createSQLException(Messages.getString("ServerPreparedStatement.8"), "S1009", this.getExceptionInterceptor()); } else { --parameterIndex; if (parameterIndex >= 0 && parameterIndex < this.parameterBindings.length) { if (this.parameterBindings[parameterIndex] == null) { this.parameterBindings[parameterIndex] = new ServerPreparedStatement.BindValue(); } else if (this.parameterBindings[parameterIndex].isLongData && !forLongData) { this.detectedLongParameterSwitch = true; } this.parameterBindings[parameterIndex].isSet = true; this.parameterBindings[parameterIndex].boundBeforeExecutionNum = (long)this.numberOfExecutions; //依据参数下标从parameterBindings数组总猎取BindValue return this.parameterBindings[parameterIndex]; } else { throw SQLError.createSQLException(Messages.getString("ServerPreparedStatement.9") + (parameterIndex + 1) + Messages.getString("ServerPreparedStatement.10") + this.parameterBindings.length, "S1009", this.getExceptionInterceptor()); } } }
就是依据参数下标从ServerPreparedStatement的参数数组parameterBindings中猎取BindValue对象，然后设置值，好了如今ServerPreparedStatement包含了预编译SQL语句的Id和参数数组，末了一步就是实行SQL了。
实行查询
实行查询操纵就是我们文章开首的末了一行代码，以下
return handler.<E>query(stmt, resultHandler);
我们来看看query是怎样做的
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException { PreparedStatement ps = (PreparedStatement)statement; //直接实行ServerPreparedStatement的execute要领 ps.execute(); return this.resultSetHandler.handleResultSets(ps); } public boolean execute() throws SQLException { this.checkClosed(); ConnectionImpl locallyScopedConn = this.connection; if (!this.checkReadOnlySafeStatement()) { throw SQLError.createSQLException(Messages.getString("PreparedStatement.20") + Messages.getString("PreparedStatement.21"), "S1009", this.getExceptionInterceptor()); } else { ResultSetInternalMethods rs = null; CachedResultSetMetaData cachedMetadata = null; synchronized(locallyScopedConn.getMutex()) { //略.... rs = this.executeInternal(rowLimit, sendPacket, doStreaming, this.firstCharOfStmt == 'S', metadataFromCache, false); //略.... } return rs != null && rs.reallyResult(); } }
省略了许多代码，只看最症结的executeInternal
ServerPreparedStatement
protected ResultSetInternalMethods executeInternal(int maxRowsToRetrieve, Buffer sendPacket, boolean createStreamingResultSet, boolean queryIsSelectOnly, Field[] metadataFromCache, boolean isBatch) throws SQLException { try { return this.serverExecute(maxRowsToRetrieve, createStreamingResultSet, metadataFromCache); } catch (SQLException var11) { throw sqlEx; } } private ResultSetInternalMethods serverExecute(int maxRowsToRetrieve, boolean createStreamingResultSet, Field[] metadataFromCache) throws SQLException { synchronized(this.connection.getMutex()) { //略.... MysqlIO mysql = this.connection.getIO(); Buffer packet = mysql.getSharedSendPacket(); packet.clear(); packet.writeByte((byte)MysqlDefs.COM_EXECUTE); //将该语句对应的id写入数据包 packet.writeLong(this.serverStatementId); int i; //将对应的参数写入数据包 for(i = 0; i < this.parameterCount; ++i) { if (!this.parameterBindings[i].isLongData) { if (!this.parameterBindings[i].isNull) { this.storeBinding(packet, this.parameterBindings[i], mysql); } else { nullBitsBuffer[i / 8] = (byte)(nullBitsBuffer[i / 8] | 1 << (i & 7)); } } } //发送数据包,示意实行id对应的预编译sql Buffer resultPacket = mysql.sendCommand(MysqlDefs.COM_EXECUTE, (String)null, packet, false, (String)null, 0); //略.... ResultSetImpl rs = mysql.readAllResults(this, this.resultSetType, resultPacket, true, (long)this.fieldCount, metadataFromCache); //返回效果 return rs; } }
ServerPreparedStatement在纪录下serverStatementId后，关于雷同SQL模板的操纵，每次只是发送serverStatementId和对应的参数，省去了编译sql的历程。 至此我们的已从数据库拿到了查询效果，然则效果是ResultSetImpl范例，我们还须要将返回效果转化成我们的java对象呢，留在下一篇来讲吧
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