MyBatis源碼解析6-SpringManagedTransaction與TransactionSynchronizationManager

SpringManagedTransaction實現了Transaction接口。裏面用到了一個很重要的類TransactionSynchronizationManager .

主要用到了TransactionSynchronizationManager的兩個方法

public static void bindResource(Object key, Object value) throws IllegalStateException;

public static Object getResource(Object key);

TransactionSynchronizationManager內部有個

	private static final ThreadLocal<Map<Object, Object>> resources =
			new NamedThreadLocal<>("Transactional resources");

每個線程都會持有一個Map<Object, Object>類型的獨立數據。
getResource(key)會根據key生成一個actualKey。然後map.put(actualKey, value), 存到這個線程持有的map裏去。

package org.mybatis.spring.transaction;

import static org.springframework.util.Assert.notNull;

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

import javax.sql.DataSource;

import org.apache.ibatis.logging.Log;
import org.apache.ibatis.logging.LogFactory;
import org.apache.ibatis.transaction.Transaction;
import org.springframework.jdbc.datasource.ConnectionHolder;
import org.springframework.jdbc.datasource.DataSourceUtils;
import org.springframework.transaction.support.TransactionSynchronizationManager;

/**
 * {@code SpringManagedTransaction} handles the lifecycle of a JDBC connection.
 * It retrieves a connection from Spring's transaction manager and returns it back to it
 * when it is no longer needed.
 * <p>
 * If Spring's transaction handling is active it will no-op all commit/rollback/close calls
 * assuming that the Spring transaction manager will do the job.
 * <p>
 * If it is not it will behave like {@code JdbcTransaction}.
 *
 * @author Hunter Presnall
 * @author Eduardo Macarron
 */
public class SpringManagedTransaction implements Transaction {

  private static final Log LOGGER = LogFactory.getLog(SpringManagedTransaction.class);

  private final DataSource dataSource;

  private Connection connection;

  private boolean isConnectionTransactional;

  private boolean autoCommit;

  public SpringManagedTransaction(DataSource dataSource) {
    notNull(dataSource, "No DataSource specified");
    this.dataSource = dataSource;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public Connection getConnection() throws SQLException {
    if (this.connection == null) {
      openConnection();
    }
    return this.connection;
  }

  /**
   * Gets a connection from Spring transaction manager and discovers if this
   * {@code Transaction} should manage connection or let it to Spring.
   * <p>
   * It also reads autocommit setting because when using Spring Transaction MyBatis
   * thinks that autocommit is always false and will always call commit/rollback
   * so we need to no-op that calls.
   */
  private void openConnection() throws SQLException {
    this.connection = DataSourceUtils.getConnection(this.dataSource);
    this.autoCommit = this.connection.getAutoCommit();
    this.isConnectionTransactional = DataSourceUtils.isConnectionTransactional(this.connection, this.dataSource);

    if (LOGGER.isDebugEnabled()) {
      LOGGER.debug(
          "JDBC Connection ["
              + this.connection
              + "] will"
              + (this.isConnectionTransactional ? " " : " not ")
              + "be managed by Spring");
    }
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public void commit() throws SQLException {
    if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) {
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Committing JDBC Connection [" + this.connection + "]");
      }
      this.connection.commit();
    }
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public void rollback() throws SQLException {
    if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) {
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Rolling back JDBC Connection [" + this.connection + "]");
      }
      this.connection.rollback();
    }
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public void close() throws SQLException {
    DataSourceUtils.releaseConnection(this.connection, this.dataSource);
  }
    
  /**
   * {@inheritDoc}
   */
  @Override
  public Integer getTimeout() throws SQLException {
    ConnectionHolder holder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
    if (holder != null && holder.hasTimeout()) {
      return holder.getTimeToLiveInSeconds();
    } 
    return null;
  }

}

我們重點看

 @Override
  public Connection getConnection() throws SQLException {
    if (this.connection == null) {
      openConnection();
    }
    return this.connection;
  }

openConnection一路查看定義，經過若干層的調用。最終。

  private void openConnection() throws SQLException {
    this.connection = DataSourceUtils.getConnection(this.dataSource);
    ...
    }

public static Connection doGetConnection(DataSource dataSource) throws SQLException {
        Assert.notNull(dataSource, "No DataSource specified");
        ConnectionHolder conHolder = (ConnectionHolder)TransactionSynchronizationManager.getResource(dataSource);
        if (conHolder == null || !conHolder.hasConnection() && !conHolder.isSynchronizedWithTransaction()) {
            logger.debug("Fetching JDBC Connection from DataSource");
            Connection con = fetchConnection(dataSource);
            if (TransactionSynchronizationManager.isSynchronizationActive()) {
                try {
                    ConnectionHolder holderToUse = conHolder;
                    if (conHolder == null) {
                        holderToUse = new ConnectionHolder(con);
                    } else {
                        conHolder.setConnection(con);
                    }

                    holderToUse.requested();
                    TransactionSynchronizationManager.registerSynchronization(new DataSourceUtils.ConnectionSynchronization(holderToUse, dataSource));
                    holderToUse.setSynchronizedWithTransaction(true);
                    if (holderToUse != conHolder) {
                        TransactionSynchronizationManager.bindResource(dataSource, holderToUse);
                    }
                } catch (RuntimeException var4) {
                    releaseConnection(con, dataSource);
                    throw var4;
                }
            }

            return con;
        } else {
            conHolder.requested();
            if (!conHolder.hasConnection()) {
                logger.debug("Fetching resumed JDBC Connection from DataSource");
                conHolder.setConnection(fetchConnection(dataSource));
            }

            return conHolder.getConnection();
        }
    }

改成僞代碼如下(只保留我們的關注點)

public static Connection doGetConnection(DataSource dataSource) throws SQLException {
        ConnectionHolder conHolder = (ConnectionHolder)TransactionSynchronizationManager.getResource(dataSource);
        if (conHolder == null || !conHolder.hasConnection() && !conHolder.isSynchronizedWithTransaction()) {
            logger.debug("Fetching JDBC Connection from DataSource");
            Connection con = fetchConnection(dataSource);
           	ConnectionHolder holderToUser = con ==> ConnectionHolder;
			TransactionSynchronizationManager.bindResource(dataSource, holderToUse);  
            return con;
        } else {
            conHolder.requested();
            if (!conHolder.hasConnection()) {
                logger.debug("Fetching resumed JDBC Connection from DataSource");
                conHolder.setConnection(fetchConnection(dataSource));
            }

            return conHolder.getConnection();
        }
    }

這裏的確用到了TransactionSynchronizationManager的兩個static方法

public static void bindResource(Object key, Object value) throws IllegalStateException;

public static Object getResource(Object key);

第一個是設置一個， 第二個是獲取一個當前線程的Map<Object k, Object v>
k設置的是DataSource， v設置的是ConnectionHolder.