Mybatis MappedStatement类核心原理详解

public final class MappedStatement {
  private String id;
  private Integer fetchSize;
  private Integer timeout;
  private StatementType statementType;
  private ResultSetType resultSetType;
  private SqlSource sqlSource;
  private Cache cache;
  private ParameterMap parameterMap;
  private List<ResultMap> resultMaps;
  private boolean flushCacheRequired;
  private boolean useCache;
  private boolean resultOrdered;
  private SqlCommandType sqlCommandType;
  private KeyGenerator keyGenerator;
  private String[] keyProperties;
  private String[] keyColumns;
  private boolean hasNestedResultMaps;
  private String databaseId;
  private Log statementLog;
  private LanguageDriver lang;
  private String[] resultSets;

statement = conn.prepareStatement(querySql,ResultSet.TYPE_FORWARD_ONLY,
                        ResultSet.CONCUR_READ_ONLY);

public static void getAll(int fetchSize) {
        try {
            long beginTime=System.currentTimeMillis();
            Connection connection = DriverManager.getConnection(MYSQL_URL);
            connection.setAutoCommit(false); //为了设置fetchSize,必须设置为false
            String sql = "select * from test";
            PreparedStatement psst = connection.prepareStatement(sql,ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);
            psst.setFetchSize(fetchSize);
            ResultSet rs = psst.executeQuery();
            int totalCount=0;
            while (rs.next()) {
                    totalCount++;
            }
            rs.close();
            psst.close();
            connection.close();
            long endTime=System.currentTimeMillis();
            System.out.println("totalCount:"+totalCount+";fetchSize:"+fetchSize+";耗时:"+(endTime-beginTime)+"ms");
        } catch (SQLException e) {
            e.printStackTrace();
        } 
    }

  public class XMLMapperBuilder extends BaseBuilder {
  private final XPathParser parser;
  private final MapperBuilderAssistant builderAssistant;
  //省略部分字段和方法
  public XMLMapperBuilder(InputStream inputStream, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {
    this(new XPathParser(inputStream, true, configuration.getVariables(), new XMLMapperEntityResolver()),
        configuration, resource, sqlFragments);
  }
  private XMLMapperBuilder(XPathParser parser, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {
    super(configuration);
    this.builderAssistant = new MapperBuilderAssistant(configuration, resource);
    this.parser = parser;
    this.sqlFragments = sqlFragments;
    this.resource = resource;
  }
  //省略部分字段和方法
  private void configurationElement(XNode context) {
    try {
      String namespace = context.getStringAttribute("namespace");
      if (namespace == null || namespace.equals("")) {
        throw new BuilderException("Mapper's namespace cannot be empty");
      }
      builderAssistant.setCurrentNamespace(namespace);
      //省略部分代码
      buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
    }
  } 

 /**
     * parseStatementNode方法是对select、insert、update、delete这四类元素进行解析，大体分为三个过程：
     * 1、解析节点属性：如我们最常用的id、resultMap等；
     * 2、解析节点内的sql语句：首先把sql语句中包含的<include></include>等标签转为实际的sql语句，然后执行静态或动态节点处理；
     * 3、根据以上解析到的内容，使用builderAssistant创建MappedStatement，并加入Configuration中。
     * <p>
     * 以上过程中最关键的是第二步，它会根据实际使用的标签，把sql片段转为不同的SqlNode，以链表方式存储到SqlSource中。
     */
    public void parseStatementNode() {
        //获取标签的id属性，如selectById，对应Mapper接口中的方法名称
        String id = context.getStringAttribute("id");
        //获取databaseId属性，我们一般都没有写。
        String databaseId = context.getStringAttribute("databaseId");
        /**
         * 这段代码虽然不起眼，但是一定要进去看一下：其内部完成了对id的再次赋值，
         * 处理的方式是：id=namespace+"."+id，也就是当前Mapper的完全限定名+"."+id，
         * 比如我们之前例子中的com.raysonxin.dao.CompanyDao.selectById
         * 这也是Mapper接口中不能存在重载方法的根本原因。
         * */
        if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
            return;
        }
        /**
         * 下面这块代码会依次获取fetchSize、timeout、resultMap等属性，
         * 需要注意的是，有些属性虽然我们没有设置，但是mybatis会设置默认值，
         * 具体可以查看mybatis的官方说明。
         */
        Integer fetchSize = context.getIntAttribute("fetchSize");
        Integer timeout = context.getIntAttribute("timeout");
        String parameterMap = context.getStringAttribute("parameterMap");
        String parameterType = context.getStringAttribute("parameterType");
        Class<?> parameterTypeClass = resolveClass(parameterType);
        String resultMap = context.getStringAttribute("resultMap");
        String resultType = context.getStringAttribute("resultType");
        String lang = context.getStringAttribute("lang");
        //默认值：XMLLanguageDriver
        LanguageDriver langDriver = getLanguageDriver(lang);
        Class<?> resultTypeClass = resolveClass(resultType);
        String resultSetType = context.getStringAttribute("resultSetType");
        //默认值：PREPARED
        StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
        //默认值：DEFAULT
        ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
        String nodeName = context.getNode().getNodeName();
        SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
        boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
        boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
        boolean useCache = context.getBooleanAttribute("useCache", isSelect);
        boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);
        // Include Fragments before parsing
        /**
         * 英文注释也说了，在sql解析前处理 include 标签，比如说，我们include了BaseColumns，
         * 它会把这个include标签替换为BaseColumns内的sql内容
         * */
        XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
        includeParser.applyIncludes(context.getNode());
        // Parse selectKey after includes and remove them.
        //处理selectKey，主要针对不同的数据库引擎做处理
        processSelectKeyNodes(id, parameterTypeClass, langDriver);
        // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
        /**
         * 到了关键步骤了：就是通过这句代码完成了从xml标签到SqlSource的转换，
         * SqlSource是一个接口，这里返回的可能是DynamicSqlSource、也可能是RawSqlSource，
         * 取决于xml标签中是否包含动态元素，比如 <if test=""></if>
         * */
        SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
        String resultSets = context.getStringAttribute("resultSets");
        //下面这些是针对selectKey、KeyGenerator等进行处理，暂时跳过了。
        String keyProperty = context.getStringAttribute("keyProperty");
        String keyColumn = context.getStringAttribute("keyColumn");
        KeyGenerator keyGenerator;
        String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
        keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
        if (configuration.hasKeyGenerator(keyStatementId)) {
            keyGenerator = configuration.getKeyGenerator(keyStatementId);
        } else {
            keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
                    configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
                    ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
        }
        /**
         * 节点及属性都解析完成了，使用builderAssistant创建MappedStatement，
         * 并保存到Configuration#mappedStatements。
         * */
        builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
                fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
                resultSetTypeEnum, flushCache, useCache, resultOrdered,
                keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
    }

  private final String sql;
  private final List<ParameterMapping> parameterMappings;
  private final Object parameterObject;

public class DynamicSqlSource implements SqlSource {
  private final Configuration configuration;
  private final SqlNode rootSqlNode;
  public DynamicSqlSource(Configuration configuration, SqlNode rootSqlNode) {
    this.configuration = configuration;
    this.rootSqlNode = rootSqlNode;
  }
  @Override
  public BoundSql getBoundSql(Object parameterObject) {
    DynamicContext context = new DynamicContext(configuration, parameterObject);
    rootSqlNode.apply(context);
    SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
    Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
    SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    context.getBindings().forEach(boundSql::setAdditionalParameter);
    return boundSql;
  }
}

public class RawSqlSource implements SqlSource {
  private final SqlSource sqlSource;
  public RawSqlSource(Configuration configuration, SqlNode rootSqlNode, Class<?> parameterType) {
    this(configuration, getSql(configuration, rootSqlNode), parameterType);
  }
  public RawSqlSource(Configuration configuration, String sql, Class<?> parameterType) {
    SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
    Class<?> clazz = parameterType == null ? Object.class : parameterType;
    sqlSource = sqlSourceParser.parse(sql, clazz, new HashMap<>());
  }
  private static String getSql(Configuration configuration, SqlNode rootSqlNode) {
    DynamicContext context = new DynamicContext(configuration, null);
    rootSqlNode.apply(context);
    return context.getSql();
  }
  @Override
  public BoundSql getBoundSql(Object parameterObject) {
    return sqlSource.getBoundSql(parameterObject);
  }
}

public class StaticSqlSource implements SqlSource {
  private final String sql;
  private final List<ParameterMapping> parameterMappings;
  private final Configuration configuration;
  public StaticSqlSource(Configuration configuration, String sql) {
    this(configuration, sql, null);
  }
  public StaticSqlSource(Configuration configuration, String sql, List<ParameterMapping> parameterMappings) {
    this.sql = sql;
    this.parameterMappings = parameterMappings;
    this.configuration = configuration;
  }
  @Override
  public BoundSql getBoundSql(Object parameterObject) {
    return new BoundSql(configuration, sql, parameterMappings, parameterObject);
  }