MyBatis加载映射文件和动态代理的实现

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <settings>
        <setting name="useGeneratedKeys" value="true"/>
    </settings>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/test?characterEncoding=utf-8&amp;serverTimezone=UTC&amp;useSSL=false"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <package name="com.mybatis.learn.dao"/>
    </mappers>
</configuration>

private void parseConfiguration(XNode root) {
    try {
        propertiesElement(root.evalNode("properties"));
        Properties settings = settingsAsProperties(root.evalNode("settings"));
        loadCustomVfs(settings);
        loadCustomLogImpl(settings);
        typeAliasesElement(root.evalNode("typeAliases"));
        pluginElement(root.evalNode("plugins"));
        objectFactoryElement(root.evalNode("objectFactory"));
        objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
        reflectorFactoryElement(root.evalNode("reflectorFactory"));
        settingsElement(settings);
        environmentsElement(root.evalNode("environments"));
        databaseIdProviderElement(root.evalNode("databaseIdProvider"));
        typeHandlerElement(root.evalNode("typeHandlers"));
        // 根据mappers标签的属性，找到映射文件/映射接口并解析
        mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
        throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
}

private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
        for (XNode child : parent.getChildren()) {
            if ("package".equals(child.getName())) {
                // 处理package子节点
                String mapperPackage = child.getStringAttribute("name");
                configuration.addMappers(mapperPackage);
            } else {
                String resource = child.getStringAttribute("resource");
                String url = child.getStringAttribute("url");
                String mapperClass = child.getStringAttribute("class");
                if (resource != null && url == null && mapperClass == null) {
                    // 处理设置了resource属性的mapper子节点
                    ErrorContext.instance().resource(resource);
                    InputStream inputStream = Resources.getResourceAsStream(resource);
                    XMLMapperBuilder mapperParser = new XMLMapperBuilder(
                            inputStream, configuration, resource, configuration.getSqlFragments());
                    mapperParser.parse();
                } else if (resource == null && url != null && mapperClass == null) {
                    // 处理设置了url属性的mapper子节点
                    ErrorContext.instance().resource(url);
                    InputStream inputStream = Resources.getUrlAsStream(url);
                    XMLMapperBuilder mapperParser = new XMLMapperBuilder(
                            inputStream, configuration, url, configuration.getSqlFragments());
                    mapperParser.parse();
                } else if (resource == null && url == null && mapperClass != null) {
                    // 处理设置了class属性的mapper子节点
                    Class<?> mapperInterface = Resources.classForName(mapperClass);
                    configuration.addMapper(mapperInterface);
                } else {
                    // 同时设置了mapper子节点的两个及以上的属性时，报错
                    throw new BuilderException(
                            "A mapper element may only specify a url, resource or class, but not more than one.");
                }
            }
        }
    }
}

public void addMappers(String packageName) {
    mapperRegistry.addMappers(packageName);
}

public void addMappers(String packageName) {
    addMappers(packageName, Object.class);
}

public void addMappers(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    // 获取包路径下的映射接口的Class对象
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
        addMapper(mapperClass);
    }
}

public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
        // 判断knownMappers中是否已经有当前映射接口
        // knownMappers是一个map存储结构，key为映射接口Class对象，value为MapperProxyFactory
        // MapperProxyFactory为映射接口对应的动态代理工厂
        if (hasMapper(type)) {
            throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
        }
        boolean loadCompleted = false;
        try {
            knownMappers.put(type, new MapperProxyFactory<>(type));
            // 依靠MapperAnnotationBuilder来完成映射文件和映射接口中的Sql解析
            // 先解析映射文件，再解析映射接口
            MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
            parser.parse();
            loadCompleted = true;
        } finally {
            if (!loadCompleted) {
                knownMappers.remove(type);
            }
        }
    }
}

public class MapperProxyFactory<T> {
    private final Class<T> mapperInterface;
    private final Map<Method, MapperMethodInvoker> methodCache = new ConcurrentHashMap<>();
    public MapperProxyFactory(Class<T> mapperInterface) {
        this.mapperInterface = mapperInterface;
    }
    public Class<T> getMapperInterface() {
        return mapperInterface;
    }
    public Map<Method, MapperMethodInvoker> getMethodCache() {
        return methodCache;
    }
    @SuppressWarnings("unchecked")
    protected T newInstance(MapperProxy<T> mapperProxy) {
        return (T) Proxy.newProxyInstance(
                mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
    }
    public T newInstance(SqlSession sqlSession) {
        final MapperProxy<T> mapperProxy = new MapperProxy<>(
                sqlSession, mapperInterface, methodCache);
        return newInstance(mapperProxy);
    }
}

public void parse() {
    String resource = type.toString();
    // 判断映射接口是否解析过，没解析过才继续往下执行
    if (!configuration.isResourceLoaded(resource)) {
        // 先解析映射文件中的Sql语句
        loadXmlResource();
        // 将当前映射接口添加到缓存中，以表示当前映射接口已经被解析过
        configuration.addLoadedResource(resource);
        assistant.setCurrentNamespace(type.getName());
        parseCache();
        parseCacheRef();
        // 解析映射接口中的Sql语句
        for (Method method : type.getMethods()) {
            if (!canHaveStatement(method)) {
                continue;
            }
            if (getAnnotationWrapper(method, false, Select.class, SelectProvider.class).isPresent()
                    && method.getAnnotation(ResultMap.class) == null) {
                parseResultMap(method);
            }
            try {
                parseStatement(method);
            } catch (IncompleteElementException e) {
                configuration.addIncompleteMethod(new MethodResolver(this, method));
            }
        }
    }
    parsePendingMethods();
}

private void loadXmlResource() {
    if (!configuration.isResourceLoaded("namespace:" + type.getName())) {
        // 根据映射接口的全限定名拼接成映射文件的路径
        // 这也解释了为什么要求映射文件和映射接口在同一目录
        String xmlResource = type.getName().replace('.', '/') + ".xml";
        InputStream inputStream = type.getResourceAsStream("/" + xmlResource);
        if (inputStream == null) {
            try {
                inputStream = Resources.getResourceAsStream(type.getClassLoader(), xmlResource);
            } catch (IOException e2) {
            }
        }
        if (inputStream != null) {
            XMLMapperBuilder xmlParser = new XMLMapperBuilder(inputStream, assistant.getConfiguration(), 
                    xmlResource, configuration.getSqlFragments(), type.getName());
            // 解析映射文件
            xmlParser.parse();
        }
    }
}

public void parse() {
    if (!configuration.isResourceLoaded(resource)) {
        // 从映射文件的<mapper>标签开始进行解析
        // 解析结果会丰富进Configuration
        configurationElement(parser.evalNode("/mapper"));
        configuration.addLoadedResource(resource);
        bindMapperForNamespace();
    }
    parsePendingResultMaps();
    parsePendingCacheRefs();
    parsePendingStatements();
}

private void configurationElement(XNode context) {
    try {
        String namespace = context.getStringAttribute("namespace");
        if (namespace == null || namespace.isEmpty()) {
            throw new BuilderException("Mapper's namespace cannot be empty");
        }
        builderAssistant.setCurrentNamespace(namespace);
        cacheRefElement(context.evalNode("cache-ref"));
        cacheElement(context.evalNode("cache"));
        // 解析<parameterMap>标签生成ParameterMap并缓存到Configuration
        parameterMapElement(context.evalNodes("/mapper/parameterMap"));
        // 解析<resultMap>标签生成ResultMap并缓存到Configuration
        resultMapElements(context.evalNodes("/mapper/resultMap"));
        // 将<sql>标签对应的节点XNode保存到sqlFragments中
        // 实际也是保存到Configuration的sqlFragments缓存中
        sqlElement(context.evalNodes("/mapper/sql"));
        // 解析<select>，<insert>，<update>和<delete>标签
        // 生成MappedStatement并缓存到Configuration
        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);
    }
}

private void buildStatementFromContext(List<XNode> list) {
    if (configuration.getDatabaseId() != null) {
        buildStatementFromContext(list, configuration.getDatabaseId());
    }
    buildStatementFromContext(list, null);
}
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    // 每一个<select>，<insert>，<update>和<delete>标签均会被创建一个MappedStatement
    // 每个MappedStatement会存放在Configuration的mappedStatements缓存中
    // mappedStatements是一个map，键为映射接口全限定名+"."+标签id，值为MappedStatement
    for (XNode context : list) {
        final XMLStatementBuilder statementParser = new XMLStatementBuilder(
                    configuration, builderAssistant, context, requiredDatabaseId);
        try {
            statementParser.parseStatementNode();
        } catch (IncompleteElementException e) {
            configuration.addIncompleteStatement(statementParser);
        }
    }
}

public void parseStatementNode() {
    // 获取标签id
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");
    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
        return;
    }
    String nodeName = context.getNode().getNodeName();
    // 获取标签的类型，例如SELECT，INSERT等
    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);
    // 如果使用了<include>标签，则将<include>标签替换为匹配的<sql>标签中的Sql片段
    // 匹配规则是在Configuration中根据namespace+"."+refid去匹配<sql>标签
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());
    // 获取输入参数类型
    String parameterType = context.getStringAttribute("parameterType");
    Class<?> parameterTypeClass = resolveClass(parameterType);
    // 获取LanguageDriver以支持实现动态Sql
    // 这里获取到的实际上为XMLLanguageDriver
    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);
    processSelectKeyNodes(id, parameterTypeClass, langDriver);
    // 获取KeyGenerator
    KeyGenerator keyGenerator;
    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
    // 先从缓存中获取KeyGenerator
    if (configuration.hasKeyGenerator(keyStatementId)) {
        keyGenerator = configuration.getKeyGenerator(keyStatementId);
    } else {
        // 缓存中如果获取不到，则根据useGeneratedKeys的配置决定是否使用KeyGenerator
        // 如果要使用，则MyBatis中使用的KeyGenerator为Jdbc3KeyGenerator
        keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
            configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
            ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
    }
    // 通过XMLLanguageDriver创建SqlSource，可以理解为Sql语句
    // 如果使用到了<if>，<foreach>等标签进行动态Sql语句的拼接，则创建出来的SqlSource为DynamicSqlSource
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    StatementType statementType = StatementType
            .valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
    // 获取<select>，<insert>，<update>和<delete>标签上的属性
    Integer fetchSize = context.getIntAttribute("fetchSize");
    Integer timeout = context.getIntAttribute("timeout");
    String parameterMap = context.getStringAttribute("parameterMap");
    String resultType = context.getStringAttribute("resultType");
    Class<?> resultTypeClass = resolveClass(resultType);
    String resultMap = context.getStringAttribute("resultMap");
    String resultSetType = context.getStringAttribute("resultSetType");
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
    if (resultSetTypeEnum == null) {
        resultSetTypeEnum = configuration.getDefaultResultSetType();
    }
    String keyProperty = context.getStringAttribute("keyProperty");
    String keyColumn = context.getStringAttribute("keyColumn");
    String resultSets = context.getStringAttribute("resultSets");
    // 根据上面获取到的参数，创建MappedStatement并添加到Configuration中
    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
        resultSetTypeEnum, flushCache, useCache, resultOrdered,
        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}

public MappedStatement addMappedStatement(
        String id,
        SqlSource sqlSource,
        StatementType statementType,
        SqlCommandType sqlCommandType,
        Integer fetchSize,
        Integer timeout,
        String parameterMap,
        Class<?> parameterType,
        String resultMap,
        Class<?> resultType,
        ResultSetType resultSetType,
        boolean flushCache,
        boolean useCache,
        boolean resultOrdered,
        KeyGenerator keyGenerator,
        String keyProperty,
        String keyColumn,
        String databaseId,
        LanguageDriver lang,
        String resultSets) {
    if (unresolvedCacheRef) {
        throw new IncompleteElementException("Cache-ref not yet resolved");
    }
    // 拼接出MappedStatement的唯一标识
    // 规则是namespace+"."+id
    id = applyCurrentNamespace(id, false);
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    MappedStatement.Builder statementBuilder = new MappedStatement
        .Builder(configuration, id, sqlSource, sqlCommandType)
            .resource(resource)
            .fetchSize(fetchSize)
            .timeout(timeout)
            .statementType(statementType)
            .keyGenerator(keyGenerator)
            .keyProperty(keyProperty)
            .keyColumn(keyColumn)
            .databaseId(databaseId)
            .lang(lang)
            .resultOrdered(resultOrdered)
            .resultSets(resultSets)
            .resultMaps(getStatementResultMaps(resultMap, resultType, id))
            .resultSetType(resultSetType)
            .flushCacheRequired(valueOrDefault(flushCache, !isSelect))
            .useCache(valueOrDefault(useCache, isSelect))
            .cache(currentCache);
    ParameterMap statementParameterMap = getStatementParameterMap(
            parameterMap, parameterType, id);
    if (statementParameterMap != null) {
        statementBuilder.parameterMap(statementParameterMap);
    }
    // 创建MappedStatement
    MappedStatement statement = statementBuilder.build();
    // 将MappedStatement添加到Configuration中
    configuration.addMappedStatement(statement);
    return statement;
}

public class MapperProxyFactory<T> {
    private final Class<T> mapperInterface;
    private final Map<Method, MapperMethodInvoker> methodCache = new ConcurrentHashMap<>();
    public MapperProxyFactory(Class<T> mapperInterface) {
        this.mapperInterface = mapperInterface;
    }
    public Class<T> getMapperInterface() {
        return mapperInterface;
    }
    public Map<Method, MapperMethodInvoker> getMethodCache() {
        return methodCache;
    }
    @SuppressWarnings("unchecked")
    protected T newInstance(MapperProxy<T> mapperProxy) {
        return (T) Proxy.newProxyInstance(
                mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
    }
    public T newInstance(SqlSession sqlSession) {
        final MapperProxy<T> mapperProxy = new MapperProxy<>(
                sqlSession, mapperInterface, methodCache);
        return newInstance(mapperProxy);
    }
}

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
        if (Object.class.equals(method.getDeclaringClass())) {
            return method.invoke(this, args);
        } else {
            // 从methodCache中根据方法对象获取MapperMethodInvoker来执行Sql
            // 如果获取不到，则创建一个MapperMethodInvoker并添加到methodCache中，再执行Sql
            return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
        }
    } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
    }
}

private MapperMethodInvoker cachedInvoker(Method method) throws Throwable {
    try {
        MapperProxy.MapperMethodInvoker invoker = methodCache.get(method);
        // 从methodCache缓存中获取到MapperMethodInvoker不为空则直接返回
        if (invoker != null) {
            return invoker;
        }
        // 从methodCache缓存中获取到MapperMethodInvoker为空
        // 则创建一个MapperMethodInvoker然后添加到methodCache缓存，并返回
        return methodCache.computeIfAbsent(method, m -> {
            // JDK1.8接口中的default()方法处理逻辑
            if (m.isDefault()) {
                try {
                    if (privateLookupInMethod == null) {
                        return new MapperProxy.DefaultMethodInvoker(getMethodHandleJava8(method));
                    } else {
                        return new MapperProxy.DefaultMethodInvoker(getMethodHandleJava9(method));
                    }
                } catch (IllegalAccessException | InstantiationException | InvocationTargetException
                        | NoSuchMethodException e) {
                    throw new RuntimeException(e);
                }
            } else {
                // 先创建一个MapperMethod
                // 再将MapperMethod作为参数创建PlainMethodInvoker
                return new MapperProxy.PlainMethodInvoker(
                    new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
            }
        });
    } catch (RuntimeException re) {
        Throwable cause = re.getCause();
        throw cause == null ? re : cause;
    }
}

public PlainMethodInvoker(MapperMethod mapperMethod) {
    super();
    this.mapperMethod = mapperMethod;
}

public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
    this.command = new SqlCommand(config, mapperInterface, method);
    this.method = new MethodSignature(config, mapperInterface, method);
}

public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
    // 获取映射接口被调用方法的方法名
    final String methodName = method.getName();
    // 获取声明被调用方法的接口的Class对象
    final Class<?> declaringClass = method.getDeclaringClass();
    // 获取和映射接口被调用方法关联的MappedStatement对象
    MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass,
            configuration);
    if (ms == null) {
        if (method.getAnnotation(Flush.class) != null) {
            name = null;
            type = SqlCommandType.FLUSH;
        } else {
            throw new BindingException("Invalid bound statement (not found): "
                    + mapperInterface.getName() + "." + methodName);
        }
    } else {
        // 将MappedStatement的id赋值给SqlCommand的name字段
        name = ms.getId();
        // 将MappedStatement的Sql命令类型赋值给SqlCommand的type字段
        // 比如SELECT，INSERT等
        type = ms.getSqlCommandType();
        if (type == SqlCommandType.UNKNOWN) {
            throw new BindingException("Unknown execution method for: " + name);
        }
    }
}

private MappedStatement resolveMappedStatement(Class<?> mapperInterface, String methodName,
                                               Class<?> declaringClass, Configuration configuration) {
    // 根据接口全限定名+"."+方法名拼接出MappedStatement的id
    String statementId = mapperInterface.getName() + "." + methodName;
    // 如果Configuration中缓存了statementId对应的MappedStatement，则直接返回这个MappedStatement
    // 这是递归的终止条件之一
    if (configuration.hasStatement(statementId)) {
        return configuration.getMappedStatement(statementId);
    } else if (mapperInterface.equals(declaringClass)) {
        // 当前mapperInterface已经是声明被调用方法的接口的Class对象，且未匹配到缓存的MappedStatement，返回null
        // 这是resolveMappedStatement()递归的终止条件之一
        return null;
    }
    // 递归调用
    for (Class<?> superInterface : mapperInterface.getInterfaces()) {
        if (declaringClass.isAssignableFrom(superInterface)) {
            MappedStatement ms = resolveMappedStatement(superInterface, methodName,
                    declaringClass, configuration);
            if (ms != null) {
                return ms;
            }
        }
    }
    return null;
}

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
        if (Object.class.equals(method.getDeclaringClass())) {
            return method.invoke(this, args);
        } else {
            // 从methodCache中根据方法对象获取MapperMethodInvoker来执行Sql
            // 如果获取不到，则创建一个MapperMethodInvoker并添加到methodCache中，再执行Sql
            return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
        }
    } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
    }
}

@Override
public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {
    return mapperMethod.execute(sqlSession, args);
}