mybatis源码解析6---MappedStatement解析

MappedStatement类位于mybatis包的org.apache.ibatis.mapping目录下，是一个final类型也就是说实例化之后就不允许改变

MappedStatement对象对应Mapper.xml配置文件中的一个select/update/insert/delete节点，描述的就是一条SQL语句，属性如下：

 

 1   private String resource;//mapper配置文件名，如：UserMapper.xml
 2   private Configuration configuration;//全局配置
 3   private String id;//节点的id属性加命名空间,如：com.lucky.mybatis.dao.UserMapper.selectByExample
 4   private Integer fetchSize;
 5   private Integer timeout;//超时时间
 6   private StatementType statementType;//操作SQL的对象的类型
 7   private ResultSetType resultSetType;//结果类型
 8   private SqlSource sqlSource;//sql语句
 9   private Cache cache;//缓存
10   private ParameterMap parameterMap;
11   private List resultMaps;
12   private boolean flushCacheRequired;
13   private boolean useCache;//是否使用缓存，默认为true
14   private boolean resultOrdered;//结果是否排序
15   private SqlCommandType sqlCommandType;//sql语句的类型，如select、update、delete、insert
16   private KeyGenerator keyGenerator;
17   private String[] keyProperties;
18   private String[] keyColumns;
19   private boolean hasNestedResultMaps;
20   private String databaseId;//数据库ID
21   private Log statementLog;
22   private LanguageDriver lang;
23   private String[] resultSets;

 

其中StatementType指操作SQL对象的类型，是个枚举类型，值分别为：

STATEMENT（直接操作SQL，不进行预编译），

PREPARED（预处理参数，进行预编译，获取数据），

CALLABLE（执行存储过程）

ResultSetType指返回结果集的类型，也是个枚举类型，值分别为：

FORWARD_ONLY：结果集的游标只能向下滚动

SCROLL_INSENSITIVE：结果集的游标可以上下移动，当数据库变化时当前结果集不变

SCROLL_SENSITIVE：结果集客自由滚动，数据库变化时当前结果集同步改变

言归正传，现在我们知道一个MappedStatement对象对应一个mapper.xml中的一个SQL节点，而Mapper.xml文件是初始化Configuration对象的时候进行解析加载的，则说明MappedStatement对象就是在初始化Configuration对象的时候创建的，并且是final类型不可更改。

之前我们知道Configuration对象的初始化过程，是通过XMLConfigBuilder类的parse方法进行初始化的，现在来看下是如何初始化MappedStatement对象的，Configuration对象初始化代码如下：

 1 public Configuration parse() {
 2     if (parsed) {
 3       throw new BuilderException("Each XMLConfigBuilder can only be used once.");
 4     }
 5     parsed = true;
 6     parseConfiguration(parser.evalNode("/configuration"));
 7     return configuration;
 8   }
 9 
10   private void parseConfiguration(XNode root) {
11     try {
12       Properties settings = settingsAsPropertiess(root.evalNode("settings"));
13       //issue #117 read properties first
14       propertiesElement(root.evalNode("properties"));
15       loadCustomVfs(settings);
16       typeAliasesElement(root.evalNode("typeAliases"));
17       pluginElement(root.evalNode("plugins"));
18       objectFactoryElement(root.evalNode("objectFactory"));
19       objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
20       reflectorFactoryElement(root.evalNode("reflectorFactory"));
21       settingsElement(settings);
22       // read it after objectFactory and objectWrapperFactory issue #631
23       environmentsElement(root.evalNode("environments"));
24       databaseIdProviderElement(root.evalNode("databaseIdProvider"));
25       typeHandlerElement(root.evalNode("typeHandlers"));
26       mapperElement(root.evalNode("mappers"));//MappedStatement对象的初始化
27     } catch (Exception e) {
28       throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
29     }
30   }

 

parseConfiguration方法是根据XNode对Configuration对象进行属性赋值，mapperElement方法即解析标签中的内容，mapperElement方法源码如下：

 1 private void mapperElement(XNode parent) throws Exception {
 2     //parent是Configuration配置文件中的标签
 3     if (parent != null) {
 4        //遍历标签下的所有子标签
 5       for (XNode child : parent.getChildren()) {
 6         if ("package".equals(child.getName())) {
 7           //加载package包下的所有mapper
 8           String mapperPackage = child.getStringAttribute("name");
 9           configuration.addMappers(mapperPackage);//加载packege包下的所有mapper
10         } else {
11           //按resource或url或class加载单个mapper
12           String resource = child.getStringAttribute("resource");
13           String url = child.getStringAttribute("url");
14           String mapperClass = child.getStringAttribute("class");
15           if (resource != null && url == null && mapperClass == null) {
16             ErrorContext.instance().resource(resource);
17             InputStream inputStream = Resources.getResourceAsStream(resource);
18             XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
19             mapperParser.parse();//解析xml文件流
20           } else if (resource == null && url != null && mapperClass == null) {
21             ErrorContext.instance().resource(url);
22             InputStream inputStream = Resources.getUrlAsStream(url);
23             XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
24             mapperParser.parse();//解析xml文件流
25           } else if (resource == null && url == null && mapperClass != null) {
26             Class mapperInterface = Resources.classForName(mapperClass);
27             configuration.addMapper(mapperInterface);//加载指定接口的mapper
28           } else {
29             throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
30           }
31         }
32       }
33     }
34   }

 

可以看出共有三种方法可以加载mapper，一个是批量加载指定package下所有mapper，一个是根据mapper接口路径加载指定mapper，还有一种是解析mapper.xml文件流进行加载，接下来挨个来看下；

先来看个最简单的，根据指定接口加载mapper，也就是configuration.addMapper(mapperInterface)方法，源码如下：

Configuration的addMapper方法

1 public  void addMapper(Class type) {
2     mapperRegistry.addMapper(type);
3   }

 

调用了mapperRegistry的addMapper方法。mapperRegistry是Configuration类的一个属性

protected final MapperRegistry mapperRegistry = new MapperRegistry(this);

 

根据MapperRegistry的名字可以理解为此类的作用是mapper的注册中心，用于注册mapper，MapperRegistry源码如下：

 1 package org.apache.ibatis.binding;
 2 
 3 import org.apache.ibatis.builder.annotation.MapperAnnotationBuilder;
 4 import org.apache.ibatis.io.ResolverUtil;
 5 import org.apache.ibatis.session.Configuration;
 6 import org.apache.ibatis.session.SqlSession;
 7 
 8 import java.util.Collection;
 9 import java.util.Collections;
10 import java.util.HashMap;
11 import java.util.Map;
12 import java.util.Set;
13 
14 public class MapperRegistry {
15 
16   private final Configuration config;//全局配置
17   private final Map, MapperProxyFactory> knownMappers = new HashMap, MapperProxyFactory>();//已注册的mapper集合
18 
19   public MapperRegistry(Configuration config) {
20     this.config = config;
21   }
22 
23   @SuppressWarnings("unchecked")
24   public  T getMapper(Class type, SqlSession sqlSession) {
25     final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type);
26     if (mapperProxyFactory == null) {
27       throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
28     }
29     try {
30       return mapperProxyFactory.newInstance(sqlSession);
31     } catch (Exception e) {
32       throw new BindingException("Error getting mapper instance. Cause: " + e, e);
33     }
34   }
35   //判断指定mapper是否已经存在
36   public  boolean hasMapper(Class type) {
37     return knownMappers.containsKey(type);
38   }
39 
40   //新增一个mapper
41   public  void addMapper(Class type) {
42     if (type.isInterface()) {
43       if (hasMapper(type)) {
44         throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
45       }
46       boolean loadCompleted = false;
47       try {
48         knownMappers.put(type, new MapperProxyFactory(type));
49         // It's important that the type is added before the parser is run
50         // otherwise the binding may automatically be attempted by the
51         // mapper parser. If the type is already known, it won't try.
52         MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
53         parser.parse();
54         loadCompleted = true;
55       } finally {
56         if (!loadCompleted) {
57           knownMappers.remove(type);
58         }
59       }
60     }
61   }
62 
63   //获取所有mapper集合
64   public Collection> getMappers() {
65     return Collections.unmodifiableCollection(knownMappers.keySet());
66   }
67 
68   
69   //根据package名称加载包下所有的mapper
70   public void addMappers(String packageName, Class superType) {
71     ResolverUtil> resolverUtil = new ResolverUtil>();
72     resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
73     Setextends Class>> mapperSet = resolverUtil.getClasses();
74     for (Class mapperClass : mapperSet) {
75       addMapper(mapperClass);
76     }
77   }
78 
79   //根据package批量加载mapper
80   public void addMappers(String packageName) {
81     addMappers(packageName, Object.class);
82   }
83   
84 }
85   

 

从源码可看出MapperRegistry就是Mapper的注册中心，有两个属性一个是全局配置Configuration还有一个是已经加载过的mapper集合 knownMappers

新增一个mapper的方法就是addMapper，就是向knownsMappers集合中put一条新的mapper记录，key就是mapper的类名全称，value是这个mapper的代理工厂；

分析到这里，发现Configuration对象初始化的时候会解析所有的xml文件中配置的所有mapper接口，并添加到Configuration的mapper集合knowMappers中，但是貌似还没有MappedStatement的影子，也没有看到哪里解析了mapper.xml配置。

不用急，上面源码的第52行就是了，52到54行的意思目前还没有看源码，但是先猜测下：52行是通过Configuration对象和mapper类来构造一个MapperAnnotationBuilder对象,通过字面意思是Mapper的构建类，而第53行的parse()，应该就是解析mapper.xml文件的，第54行标记加载完成，

只有当mapper接口和mapper.xml匹配成功才能叫做是加载成功，所以下一章篇就再来看看MappedStatement是如何创建的。

总结：MappedStatement类就是对应的Mapper.xml中的一个sql语句

转载于:https://www.cnblogs.com/jackion5/p/9482052.html