MyBatis源码解读(二)
MyBatis缓存
如果一级、二级缓存同时开启的话。会先去查询二级缓存再去查询一级缓存。
二级缓存的开启配置我们就不多做介绍，我们主要进行二级缓存开启后的执行流程。
1、开启二级缓存
以下代码就是我们解析配置文件的时候进行解析的代码。会解析到我们开启二级缓存的配置。
1、
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
//委托XMLConfigBuilder来解析xml文件，并构建
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
return build(parser.parse());
} catch (Exception e) {
//这里是捕获异常，包装成自己的异常并抛出的idiom？，最后还要reset ErrorContext
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
reader.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}
2、//追parser.parse()中的mapperElement(root.evalNode("mappers"));
//因为这个方法解析的是我们的Mapper配置文件。
private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
if ("package".equals(child.getName())) {
//10.4自动扫描包下所有映射器
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) {
//10.1使用类路径
ErrorContext.instance().resource(resource);
InputStream inputStream = Resources.getResourceAsStream(resource);
//映射器比较复杂，调用XMLMapperBuilder
//注意在for循环里每个mapper都重新new一个XMLMapperBuilder，来解析
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
//这个方法就开始解析映射配置文件的。
mapperParser.parse();
} else if (resource == null && url != null && mapperClass == null) {
//10.2使用绝对url路径
ErrorContext.instance().resource(url);
InputStream inputStream = Resources.getUrlAsStream(url);
//映射器比较复杂，调用XMLMapperBuilder
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
mapperParser.parse();
} else if (resource == null && url == null && mapperClass != null) {
//10.3使用java类名
Class<?> mapperInterface = Resources.classForName(mapperClass);
//直接把这个映射加入配置
configuration.addMapper(mapperInterface);
} else {
throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
}
}
}
}
}
3、//mapperParser.parse();
//解析
public void parse() {
//如果没有加载过再加载，防止重复加载
if (!configuration.isResourceLoaded(resource)) {
//开始解析mapper根节点
configurationElement(parser.evalNode("/mapper"));
//标记一下，已经加载过了
configuration.addLoadedResource(resource);
//绑定映射器到namespace
bindMapperForNamespace();
}
//还有没解析完的东东这里接着解析？
parsePendingResultMaps();
parsePendingChacheRefs();
parsePendingStatements();
}
4、//configurationElement(parser.evalNode("/mapper"));
private void configurationElement(XNode context) {
try {
//1.配置namespace
String namespace = context.getStringAttribute("namespace");
if (namespace.equals("")) {
throw new BuilderException("Mapper's namespace cannot be empty");
}
builderAssistant.setCurrentNamespace(namespace);
//2.配置cache-ref
cacheRefElement(context.evalNode("cache-ref"));
//3.配置cache 这就是我们要找的Cache。
cacheElement(context.evalNode("cache"));
//4.配置parameterMap(已经废弃,老式风格的参数映射)
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
//5.配置resultMap(高级功能)
resultMapElements(context.evalNodes("/mapper/resultMap"));
//6.配置sql(定义可重用的 SQL 代码段)
sqlElement(context.evalNodes("/mapper/sql"));
//7.配置select|insert|update|delete TODO
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
} catch (Exception e) {
throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
}
}
5、//终于找到了我们想要的Cache节点解析方法。
private void cacheElement(XNode context) throws Exception {
if (context != null) {
String type = context.getStringAttribute("type", "PERPETUAL");
Class<? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type);
String eviction = context.getStringAttribute("eviction", "LRU");
Class<? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction);
Long flushInterval = context.getLongAttribute("flushInterval");
Integer size = context.getIntAttribute("size");
boolean readWrite = !context.getBooleanAttribute("readOnly", false);
boolean blocking = context.getBooleanAttribute("blocking", false);
//读入额外的配置信息，易于第三方的缓存扩展,例:
// <cache type="com.domain.something.MyCustomCache">
// <property name="cacheFile" value="/tmp/my-custom-cache.tmp"/>
// </cache>
Properties props = context.getChildrenAsProperties();
//调用builderAssistant.useNewCache
builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
}
}
6、//解析完我们配置的参数便调用了下面：builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
public Cache useNewCache(Class<? extends Cache> typeClass,
Class<? extends Cache> evictionClass,
Long flushInterval,
Integer size,
boolean readWrite,
boolean blocking,
Properties props) {
//这里面又判断了一下是否为null就用默认值，有点和XMLMapperBuilder.cacheElement逻辑重复了
typeClass = valueOrDefault(typeClass, PerpetualCache.class);
evictionClass = valueOrDefault(evictionClass, LruCache.class);
//调用CacheBuilder构建cache,id=currentNamespace
Cache cache = new CacheBuilder(currentNamespace)
.implementation(typeClass)
.addDecorator(evictionClass)
.clearInterval(flushInterval)
.size(size)
.readWrite(readWrite)
.blocking(blocking)
.properties(props)
.build();
//将我们的缓存配置加入到了核心配置类当中
configuration.addCache(cache);
//当前的缓存
currentCache = cache;
return cache;
}
经过上面源码流程，我们得知的缓存对象的创建。但是还没完，还有一部分select、update、delete标签中的属性要解析：我们要将创建的缓存对象放入到Statement中去。
1、
private void configurationElement(XNode context) {
try {
//1.配置namespace
String namespace = context.getStringAttribute("namespace");
if (namespace.equals("")) {
throw new BuilderException("Mapper's namespace cannot be empty");
}
builderAssistant.setCurrentNamespace(namespace);
//2.配置cache-ref
cacheRefElement(context.evalNode("cache-ref"));
//3.配置cache
cacheElement(context.evalNode("cache"));
//4.配置parameterMap(已经废弃,老式风格的参数映射)
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
//5.配置resultMap(高级功能)
resultMapElements(context.evalNodes("/mapper/resultMap"));
//6.配置sql(定义可重用的 SQL 代码段)
sqlElement(context.evalNodes("/mapper/sql"));
//7.配置select|insert|update|delete TODO
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
} catch (Exception e) {
throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
}
}
2、// buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
for (XNode context : list) {
//构建所有语句,一个mapper下可以有很多select
//语句比较复杂，核心都在这里面，所以调用XMLStatementBuilder
final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
try {
//核心XMLStatementBuilder.parseStatementNode
statementParser.parseStatementNode();
} catch (IncompleteElementException e) {
//如果出现SQL语句不完整，把它记下来，塞到configuration去
configuration.addIncompleteStatement(statementParser);
}
}
}
3、//statementParser.parseStatementNode();主要是去解析select|insert|update|delete这些标签，这里面也涉及到一些缓存配置。
public void parseStatementNode() {
String id = context.getStringAttribute("id");
String databaseId = context.getStringAttribute("databaseId");
//如果databaseId不匹配，退出
if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
return;
}
//暗示驱动程序每次批量返回的结果行数
Integer fetchSize = context.getIntAttribute("fetchSize");
//超时时间
Integer timeout = context.getIntAttribute("timeout");
//引用外部 parameterMap,已废弃
String parameterMap = context.getStringAttribute("parameterMap");
//参数类型
String parameterType = context.getStringAttribute("parameterType");
Class<?> parameterTypeClass = resolveClass(parameterType);
//引用外部的 resultMap(高级功能)
String resultMap = context.getStringAttribute("resultMap");
//结果类型
String resultType = context.getStringAttribute("resultType");
//脚本语言,mybatis3.2的新功能
String lang = context.getStringAttribute("lang");
//得到语言驱动
LanguageDriver langDriver = getLanguageDriver(lang);
Class<?> resultTypeClass = resolveClass(resultType);
//结果集类型，FORWARD_ONLY|SCROLL_SENSITIVE|SCROLL_INSENSITIVE 中的一种
String resultSetType = context.getStringAttribute("resultSetType");
//语句类型, STATEMENT|PREPARED|CALLABLE 的一种
StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
//获取命令类型(select|insert|update|delete)
String nodeName = context.getNode().getNodeName();
SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
//是否要缓存select结果
boolean useCache = context.getBooleanAttribute("useCache", isSelect);
//仅针对嵌套结果 select 语句适用：如果为 true，就是假设包含了嵌套结果集或是分组了，这样的话当返回一个主结果行的时候，就不会发生有对前面结果集的引用的情况。
//这就使得在获取嵌套的结果集的时候不至于导致内存不够用。默认值：false。
boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);
// Include Fragments before parsing
//解析之前先解析<include>SQL片段
XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
includeParser.applyIncludes(context.getNode());
// Parse selectKey after includes and remove them.
//解析之前先解析<selectKey>
processSelectKeyNodes(id, parameterTypeClass, langDriver);
// Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
//解析成SqlSource，一般是DynamicSqlSource
SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
String resultSets = context.getStringAttribute("resultSets");
//(仅对 insert 有用) 标记一个属性, MyBatis 会通过 getGeneratedKeys 或者通过 insert 语句的 selectKey 子元素设置它的值
String keyProperty = context.getStringAttribute("keyProperty");
//(仅对 insert 有用) 标记一个属性, MyBatis 会通过 getGeneratedKeys 或者通过 insert 语句的 selectKey 子元素设置它的值
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))
? new Jdbc3KeyGenerator() : new NoKeyGenerator();
}
//又去调助手类，通过该方法创建MappedStatement
builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}
4、/**builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
} **/
//增加映射语句
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");
}
//为id加上namespace前缀
id = applyCurrentNamespace(id, false);
//是否是select语句
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
//又是建造者模式
MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType);
statementBuilder.resource(resource);
statementBuilder.fetchSize(fetchSize);
statementBuilder.statementType(statementType);
statementBuilder.keyGenerator(keyGenerator);
statementBuilder.keyProperty(keyProperty);
statementBuilder.keyColumn(keyColumn);
statementBuilder.databaseId(databaseId);
statementBuilder.lang(lang);
statementBuilder.resultOrdered(resultOrdered);
statementBuilder.resulSets(resultSets);
setStatementTimeout(timeout, statementBuilder);
//1.参数映射
setStatementParameterMap(parameterMap, parameterType, statementBuilder);
//2.结果映射
setStatementResultMap(resultMap, resultType, resultSetType, statementBuilder);
//将之前封装好的缓存对象currentCache设置进去。
setStatementCache(isSelect, flushCache, useCache, currentCache, statementBuilder);
MappedStatement statement = statementBuilder.build();
//建造好调用configuration.addMappedStatement
configuration.addMappedStatement(statement);
return statement;
}
2、二级缓存执行流程
2.1
既然二级缓存都已经配置好了，那我们看看二级缓存是怎样使用的呐。
我们直接在selectList入手：
//核心selectList
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
//根据statement id找到对应的MappedStatement
MappedStatement ms = configuration.getMappedStatement(statement);
//转而用执行器来查询结果,注意这里传入的ResultHandler是null,注意这个执行器，如果开启了缓存，他就是CachingExecutor
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();
}
}
我们继续追一下query方法：
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
//得到缓存对象，我们以前准备好存进去的。
Cache cache = ms.getCache();
//默认情况下是没有开启缓存的(二级缓存).要开启二级缓存,你需要在你的 SQL 映射文件中添加一行: <cache/>
//简单的说，就是先查CacheKey，查不到再委托给实际的执行器去查
if (cache != null) {
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, parameterObject, boundSql);
@SuppressWarnings("unchecked")
//从二级缓存中获取对应结果。
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
//这个方法先去执行一级缓存，delegate是simpleExecutor执行器。
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
2.2
既然已经知道了执行流程，接下来看下二级缓存具体的存取操作：
//得到某个TransactionalCache的值
public Object getObject(Cache cache, CacheKey key) {
return getTransactionalCache(cache).getObject(key);
}
//getTransactionalCache(cache)
//管理了许多TransactionalCache
private Map<Cache, TransactionalCache> transactionalCaches = new HashMap<Cache, TransactionalCache>();
private TransactionalCache getTransactionalCache(Cache cache) {
TransactionalCache txCache = transactionalCaches.get(cache);
if (txCache == null) {
txCache = new TransactionalCache(cache);
transactionalCaches.put(cache, txCache);
}
return txCache;
}
//getObject(key) 通过上面得到的TransactionalCache对象通过key去获取缓存。
@Override
public Object getObject(Object key) {
Object object = delegate.getObject(key);
if (object == null) {
entriesMissedInCache.add(key);
}
if (clearOnCommit) {
return null;
} else {
return object;
}
}
//Cache是一个接口，不同的缓存类型对象要去实现它。
2.3
不过还没完，以目前所说的并不能解决事物问题，我们去看看事物问题是如何解决的？
//缓存对象，里面存着缓存。
private Cache delegate;
//commit时要不要清缓存
private boolean clearOnCommit;
//事物被提交前，所有从数据库中查询的结果缓存在这里面
private Map<Object, Object> entriesToAddOnCommit;
//事物被提交前，当缓存未命中是，CacheKey被存在这个集合中
private Set<Object> entriesMissedInCache;
//put
@Override
public void putObject(Object key, Object object) {
//添加时不直接放入缓存对象。
entriesToAddOnCommit.put(key, object);
}
//commit
public void commit() {
//判断commit的时候判断要不要删除以前的缓存
if (clearOnCommit) {
delegate.clear();
}
flushPendingEntries();
reset();
}
1、
private void flushPendingEntries() {
for (Map.Entry<Object, Object> entry : entriesToAddOnCommit.entrySet()) {
//开始存放到真正的缓存当中，所以说不commit根本查不到
delegate.putObject(entry.getKey(), entry.getValue());
}
for (Object entry : entriesMissedInCache) {
if (!entriesToAddOnCommit.containsKey(entry)) {
//没命中且不在entriesToAddOnCommit，就是两个缓存都没有，直接赋值为空
delegate.putObject(entry, null);
}
}
}
private void unlockMissedEntries() {
for (Object entry : entriesMissedInCache) {
delegate.putObject(entry, null);
}
}
}
2、 private void reset() {
clearOnCommit = false;
entriesToAddOnCommit.clear();
entriesMissedInCache.clear();
}
MyBatis延迟加载
延迟加载就是当数据使用的时候再加载进来。例子就是用户关联订单，我们在查询时，首先查询出用户数据，当用到订单数据时再去加载。
同样，延迟加载配置开启就不做过多介绍，我们来探讨它的部分源码。
延迟加载的原理就是实现目标对象的代理对象。
UserDao userDao = sqlSession.getMapper(UserDao.class);
上图返回的userDao对象为代理对象。我们调用它关联的数据对象的get方法时就会进入拦截器，判断是否需要延迟加载，所以就进入了准备好的SQL将关联对象查出来。
1、代理对象的创建
下面展示为结果对象创建代理对象的代码。
private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, ResultLoaderMap lazyLoader, String columnPrefix) throws SQLException {
final List<Class<?>> constructorArgTypes = new ArrayList<Class<?>>();
final List<Object> constructorArgs = new ArrayList<Object>();
//创建映射后的结果对象
final Object resultObject = createResultObject(rsw, resultMap, constructorArgTypes, constructorArgs, columnPrefix);
if (resultObject != null && !typeHandlerRegistry.hasTypeHandler(resultMap.getType())) {
//如果有内嵌查询，并且开启了延迟加载，则创建结果对象的的代理对象
//得到我们再配置文件ResultMapping中配置的属性
//每一个ResultMapping对应xml文件中的一个配置。
final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
for (ResultMapping propertyMapping : propertyMappings) {
//判断ResultMapping里面子标签有没有配置延迟加载
if (propertyMapping.getNestedQueryId() != null && propertyMapping.isLazy()) {
//TODO 使用代理(cglib/javaassist)去创建队里对象。
return configuration.getProxyFactory().createProxy(resultObject, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
}
}
}
return resultObject;
}
//默认使用Javassist
//protected ProxyFactory proxyFactory = new JavassistProxyFactory();
既然找到的代理工厂我们便去看看代理工厂创建代理对象的代码。
public static Object createProxy(Object target, ResultLoaderMap lazyLoader, Configuration configuration, ObjectFactory objectFactory, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
final Class<?> type = target.getClass();
//代理类执行时就会去执行它的invoke
EnhancedResultObjectProxyImpl callback = new EnhancedResultObjectProxyImpl(type, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
//生成代理方法
Object enhanced = crateProxy(type, callback, constructorArgTypes, constructorArgs);
PropertyCopier.copyBeanProperties(type, target, enhanced);
return enhanced;
}
2、代理对象的执行
代理对象已经创建完成，我们就可以去看看代理对象的执行过程了。
@Override
public Object invoke(Object enhanced, Method method, Method methodProxy, Object[] args) throws Throwable {
final String methodName = method.getName();
try {
synchronized (lazyLoader) {
if (WRITE_REPLACE_METHOD.equals(methodName)) {
Object original = null;
if (constructorArgTypes.isEmpty()) {
original = objectFactory.create(type);
} else {
original = objectFactory.create(type, constructorArgTypes, constructorArgs);
}
PropertyCopier.copyBeanProperties(type, enhanced, original);
if (lazyLoader.size() > 0) {
return new JavassistSerialStateHolder(original, lazyLoader.getProperties(), objectFactory, constructorArgTypes, constructorArgs);
} else {
return original;
}
} else {
if (lazyLoader.size() > 0 && !FINALIZE_METHOD.equals(methodName)) {
//加载所有延迟加载的属性。
if (aggressive || lazyLoadTriggerMethods.contains(methodName)) {
lazyLoader.loadAll();
//判断必须以get|set|is开头。
} else if (PropertyNamer.isProperty(methodName)) {
//根据方法名替换为属性。
final String property = PropertyNamer.methodToProperty(methodName);
//判断这个属性是否是延迟加载的属性。
if (lazyLoader.hasLoader(property)) {
//根据延迟属性进行加载
lazyLoader.load(property);
}
}
}
}
}
//执行原方法
return methodProxy.invoke(enhanced, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
