Mybatis源碼概覽(一)
通常拿到源碼會無從下手，個人基本思路通常就是根據一個基本的helloWorld Debug下去，把主線先大概理一遍，而後再具體分析細節，沒有必要一個類一個類細看，看了也會忘掉。本身理源碼的時候看不下去時，能夠結合網上的分析文章，一邊看別人的解析，一邊本身對照源碼。瞭解框架設計原理，之後項目中出了問題能夠更容易定位。再往上一層面，之後本身能夠根據需求擴展框架。java
先執行個HelloWorldmysql
去github上 clone Mybatis代碼，而後再其測試源碼裏添加以下代碼
git
示例代碼，裏面未貼出來的類自行補全。github
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession session = sqlSessionFactory.openSession(true);
try {
//後面介紹mybatis經過動態代理來避免手工調用session,直接調用dao接口；
//BlogDao mapper = session.getMapper(BlogDao.class);
//List<Blog> blogs= mapper.selectAll();
List<Blog>blogs= session.selectList("org.apache.ibatis.robin.BlogDao.selectAll");
System.out.println(blogs);
} finally {
session.close();
}
mybatis-config.xmlspring
<?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> <environments default="development"> <environment id="development"> <transactionManager type="JDBC"/> <dataSource type="POOLED"> <property name="driver" value="com.mysql.jdbc.Driver"/> <property name="url" value="jdbc:mysql://localhost:3306/test?useUnicode=true&characterEncoding=UTF-8"/> <property name="username" value="root"/> <property name="password" value="root"/> </dataSource> </environment> </environments> <mappers> <mapper resource="mapper/BlogMapper.xml"/> </mappers> </configuration>
BlogMapper.xml
sql
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="org.apache.ibatis.robin.BlogDao">
<resultMap type="org.apache.ibatis.robin.Blog" id="blog">
<result property="id" column="id"/>
<result property="context" column="context"/>
<result property="dateCreate" column="date_create"/>
</resultMap>
<insert id="insert" parameterType="org.apache.ibatis.robin.Blog">
insert into blog(id,context,date_create)
values (#{id},#{context},now())
</insert>
<select id="selectAll" resultMap="blog" flushCache="true" >
SELECT * from blog
</select>
</mapper>
注意xml文件放到項目的resource位置，能夠經過ide來設置，不然程序會獲取不到。 數據庫
一步一步DEBUG
解析配置，構建SqlSessionFactory
從上面的代碼能夠看到mybatis重要的執行順序：輸入配置流，由SqlSessionFacotryBuilder來根據輸入的配置構建出來一個SqlSessionFactory。apache
//簡略代碼
//解析配置
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
//根據配置返回SqlSessionFactory
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
構建打開SqlSession （executor生成，plugin織入，緩存開閉）
獲取到SqlSessionFactory後，下一步確定是得到sqlSession。緩存
sqlSessionFactory.openSession(true); //這裏的true 表示是否自動commit
//debug 進去 這部分是openSession的大致過程；
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
//先經過configuration獲取咱們再xml中配的environment，裏面包含事務和DataSource;
final Environment environment = configuration.getEnvironment();
//這一步是獲取JDBC事務
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
//打開Executor過程，見下段落的分析
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
打開Executor的過程，Executor默認的類型是SIMPLEsession
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
executor = new SimpleExecutor(this, transaction);
}
//若是在配置文件settings中打開 <setting name="cacheEnabled" value="true" />
// cacheEnabled 着爲true,開啓全局緩存，也就是二級緩存；
//下面查詢具體分析CachingExecutor執行查詢過程
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
//若是自定義了Plugin攔截器，在xml經過plugins配置後，這一步會經過JDK動態代理織入到executor中，
//生成一個帶了攔截器方法功能的Executor
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
在根BaseExecutor中能夠看到 ,Executor不只包含了事務，還同時加入localCache ，也就是Session級別的緩存，也是你們常叫的一級緩存，這裏提一下一級緩存是默認的，若是非要去掉只能經過在select 語句配置中 flushCache="true"。
protected BaseExecutor(Configuration configuration, Transaction transaction) {
this.transaction = transaction;
this.deferredLoads = new ConcurrentLinkedQueue<DeferredLoad>();
this.localCache = new PerpetualCache("LocalCache");
this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
this.closed = false;
this.configuration = configuration;
this.wrapper = this;
}
Executor組裝好以後經過new DefaultSqlSession返回咱們的SqlSession;
new DefaultSqlSession(configuration, executor, autoCommit);
執行具體查詢（重要概念MappedStatement）
SqlSession是直接對數據庫發號施令的組件。經過發起下面一個SQL查詢，繼續Debug進去
List<Blog>blogs= session.selectList("org.apache.ibatis.robin.BlogDao.selectAll");
先mybatis自動補全一些默認參數（rowBounds主要是指代返回的行數限制）後，進去下面的代碼
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
//MappedStatement是Mybatis的精髓，若是Bean對Spring是同樣的道理；這裏咱們要重點介紹下
//屬性MappedStatement 是一個SQL執行語句的包裝，裏面的屬性有SqlSource（指代SQL具體的語句）；
//屬性StatementType 若是是PREPARED代表執行預編譯執行，這樣不只防止SQL注入，並且還能避免SQL重複解析；
//屬性id 指代咱們惟一肯定MappedStatement惟一標識；
//還有ResultMap 和ParameterMap等等，這裏就不解釋來；
//能夠說MappedStatement是發起SQL請求的所需的必備數據；
MappedStatement ms = configuration.getMappedStatement(statement);
//進行query查詢，轉下段分析
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
//獲取SQL
BoundSql boundSql = ms.getBoundSql(parameterObject);
//經過參數,sql,和rowBounds一塊兒拼裝出cacheKey
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
//進入到查詢
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
//先判斷是否有緩存中去取
Cache cache = ms.getCache();
if (cache != null) {
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, parameterObject, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
//緩存中取不到，直接執行query,這裏delegate指代咱們前面生成的SimpleExecutor;
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
//跳轉到下面的方法
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
if (closed) {
throw new ExecutorException("Executor was closed.");
}
//清掉cache
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
queryStack++;
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
//跳到下面到數據庫中去查詢數據
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
queryStack--;
}
if (queryStack == 0) {
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
return list;
}
//從這個方法能夠明顯看出裏面的localCache，指代前面說的Session緩存，即一級緩存
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
//這一步主要是獲取Connection,而後準備PreparedState,執行查詢返回結果
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}
//把新結果緩存起來
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}
至此咱們的查詢SQL就執行完了。
這一路涉及了 Configuration --->SqlSessionFactory------>SqlSession(裏面又包裝了 Executor,Cache,MappedStatement) 這幾個重要概念；
下節主要講Mybatis經過代理 把原先本身須要直接調用 sqlSession來執行 改爲只需調用相應dao接口類,在實際項目中省掉大量代碼，以及與spring結合的實現原理；
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