mybatis打印JDBC日志源码分析（JDK动态代理）

/**
 * Connection proxy to add logging
 * BaseJdbcLogger实现了几个代理的共同方法
 */
public final class ConnectionLogger extends BaseJdbcLogger implements InvocationHandler {

  /**
   * 动态代理的目标对象
   */
  private final Connection connection;

  private ConnectionLogger(Connection conn, Log statementLog, int queryStack) {
    super(statementLog, queryStack);
    this.connection = conn;
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] params)
      throws Throwable {
    try {
      // 返回拥有method方法的class
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, params);
      }
      /**
       * jdbc包下的这些类相当于装饰器
       * 不过这里从装饰是把返回的类都装饰成了代理类
       * 判断当前的方法是否需要代理
       */
      if ("prepareStatement".equals(method.getName()) || "prepareCall".equals(method.getName())) {
        if (isDebugEnabled()) {
          debug(" Preparing: " + removeExtraWhitespace((String) params[0]), true);
        }
        // 通过反射的方式调用代理类的代理方法
        PreparedStatement stmt = (PreparedStatement) method.invoke(connection, params);
        // PreparedStatement对返回值再次加工和封装，返回一个新的代理类
        // 代理和被代理的类都需要实现同样的接口，这样就可以发挥java多态的特性了
        stmt = PreparedStatementLogger.newInstance(stmt, statementLog, queryStack);
        return stmt;
      } else if ("createStatement".equals(method.getName())) {
        Statement stmt = (Statement) method.invoke(connection, params);
        stmt = StatementLogger.newInstance(stmt, statementLog, queryStack);
        return stmt;
      } else {
        return method.invoke(connection, params);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
  }

  /**
   * Creates a logging version of a connection.
   * 
   * 创建Connection代理的方法，注意这是一个static方法
   */
  public static Connection newInstance(Connection conn, Log statementLog, int queryStack) {
    InvocationHandler handler = new ConnectionLogger(conn, statementLog, queryStack);
    ClassLoader cl = Connection.class.getClassLoader();
    return (Connection) Proxy.newProxyInstance(cl, new Class[]{Connection.class}, handler);
  }
}