mybatis 原始碼分析(八)ResultSetHandler 詳解
本篇部落格就是 myabtis 系列的最後一篇了,還剩 ResultSetHandler 沒有分析;作為整個 mybatis 最複雜最繁瑣的部分,我不打算按步驟一次詳解,因為裡面的主要內容就是圍繞 resultMap 按層次結構依次解析的,其中運用最多的就是反射,所以我這裡將圍繞延遲載入重點分析,另外本文使用的測試程式碼都是原始碼的測試案例;
一、ResultSetHandler 主體結構
public interface ResultSetHandler { // 負責結果集處理,完成對映返回結果物件 <E> List<E> handleResultSets(Statement stmt) throws SQLException; // 負責遊標物件處理 <E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException; // 負責儲存過程的輸出引數 void handleOutputParameters(CallableStatement cs) throws SQLException; }
以上就是 ResultSetHandler 的介面方法(mybatis 中只提供了唯一的實現類 DefaultResultSetHandler),在本篇部落格中將主要以 handleResultSets 結果集處理作為主線分析;
在分析之前首先要清楚 handleResultSets 方法的處理流程就是圍繞 resultMap 依次解析的,這裡先看一個比較複雜的 resultMap 對映:
<resultMap id="detailedBlogResultMap" type="Blog"> <constructor> <idArg column="blog_id" javaType="int"/> </constructor> <result property="title" column="blog_title"/> <association property="author" javaType="Author"> <id property="id" column="author_id"/> <result property="username" column="author_username"/> <result property="password" column="author_password"/> </association> <collection property="posts" ofType="Post"> <id property="id" column="post_id"/> <result property="subject" column="post_subject"/> <association property="author" javaType="Author"/> <collection property="comments" ofType="Comment"> <id property="id" column="comment_id"/> </collection> <discriminator javaType="int" column="draft"> <case value="1" resultType="DraftPost"/> </discriminator> </collection> <association property="author" column="id" select="**.selectAuthorForBlog" fetchType="eager"/> <collection property="posts" javaType="ArrayList" column="id" ofType="Post" select="**.selectPostsForBlog" fetchType="lazy"/> </resultMap>
當 mybatis 初始化完成後上面的配置都放到 MappedStatement.resultMaps 裡面,在解析的時候就是通過 resultMap.id 取到對應的 resultMap 然後逐次解析;
1. 巢狀查詢
這裡之所以說 ResultSetHandler 是整個 mybatis 裡面最複雜的,主要是巢狀查詢的解析(association 一對一,collection 一對多),值得注意的是這裡的巢狀查詢是有兩種方式的:
內部巢狀
<association property="author" javaType="Author"> <id property="id" column="author_id"/> <result property="username" column="author_username"/> <result property="password" column="author_password"/> </association> <collection property="posts" ofType="Post"> <id property="id" column="post_id"/> <result property="subject" column="post_subject"/> <association property="author" javaType="Author"/> <collection property="comments" ofType="Comment"> <id property="id" column="comment_id"/> </collection> <discriminator javaType="int" column="draft"> <case value="1" resultType="DraftPost"/> </discriminator> </collection>
像這種巢狀查詢是直接在同一個 resultMap 依次對映對應結果的,使用的是 SQL 多表連線,例如:
<select id="selectBlogDetails" resultMap="detailedBlogResultMap">
select
B.id as blog_id,
B.title as blog_title,
B.author_id as blog_author_id,
A.id as author_id,
A.username as author_username,
...
P.id as post_id,
P.blog_id as post_blog_id,
...
C.id as comment_id,
C.post_id as comment_post_id,
...
T.id as tag_id,
T.name as tag_name
from Blog B
left outer join Author A on B.author_id = A.id
left outer join Post P on B.id = P.blog_id
left outer join Comment C on P.id = C.post_id
left outer join Post_Tag PT on PT.post_id = P.id
left outer join Tag T on PT.tag_id = T.id
where B.id = #{id}
</select>
這裡還有一種分離的內部巢狀:
<resultMap id="blogResult" type="Blog">
<id property="id" column="blog_id" />
<result property="title" column="blog_title"/>
<association property="author" column="blog_author_id" javaType="Author" resultMap="authorResult"/>
</resultMap>
<resultMap id="authorResult" type="Author">
<id property="id" column="author_id"/>
<result property="username" column="author_username"/>
<result property="password" column="author_password"/>
<result property="email" column="author_email"/>
<result property="bio" column="author_bio"/>
</resultMap>
<select id="selectBlog" resultMap="blogResult">
select
B.id as blog_id,
B.title as blog_title,
B.author_id as blog_author_id,
A.id as author_id,
A.username as author_username,
A.password as author_password,
A.email as author_email,
A.bio as author_bio
from Blog B left outer join Author A on B.author_id = A.id
where B.id = #{id}
</select>
這中寫法只是將 association、collection 部分分離出來,其實質都是一樣的,都是一條多表連線的 SQL;
外部巢狀
還有另外一種是將多表連線的 SQL 拆分,每個屬性單獨發一條 SQL:
<resultMap id="blogResult" type="Blog">
<collection property="posts" javaType="ArrayList" column="id" ofType="Post" select="selectPostsForBlog"/>
</resultMap>
<select id="selectBlog" resultMap="blogResult">
SELECT * FROM BLOG WHERE ID = #{id}
</select>
<select id="selectPostsForBlog" resultType="Post">
SELECT * FROM POST WHERE BLOG_ID = #{id}
</select>
像這樣在 association、collection 中使用 select 屬性指定外部 SQL,其查詢結果也是發兩條 SQL,這裡之所以沒有詳細寫出每個屬性的對映,是因為指定了 type 和 ofType,並開啟的自動對映,mybatis 在執行的時候使用反射推斷出來的;
這裡的兩種巢狀查詢在初始化的時候就進行了單獨的區分:
// org.apache.ibatis.builder.xml.XMLMapperBuilder
String nestedResultMap = context.getStringAttribute("resultMap", processNestedResultMappings(context, Collections.emptyList(), resultType));
// org.apache.ibatis.submitted.nestedresulthandler.Mapper.mapper_resultMap[personResult]_collection[items]
private String processNestedResultMappings(XNode context, List<ResultMapping> resultMappings, Class<?> enclosingType) throws Exception {
if ("association".equals(context.getName())
|| "collection".equals(context.getName())
|| "case".equals(context.getName())) {
if (context.getStringAttribute("select") == null) {
validateCollection(context, enclosingType);
ResultMap resultMap = resultMapElement(context, resultMappings, enclosingType);
return resultMap.getId();
}
}
return null;
}
// org.apache.ibatis.mapping.ResultMap
// resultMap.hasNestedResultMaps = resultMap.hasNestedResultMaps || (resultMapping.getNestedResultMapId() != null && resultMapping.getResultSet() == null);
這裡程式碼程式碼比較多所以只放了關鍵程式碼,其最終結果是內部巢狀由 ResultMap.hasNestedResultMaps 標識;
// org.apache.ibatis.builder.xml.XMLMapperBuilder
private ResultMapping buildResultMappingFromContext(XNode context, Class<?> resultType, List<ResultFlag> flags) throws Exception {
...
String nestedSelect = context.getStringAttribute("select");
String nestedResultMap = context.getStringAttribute("resultMap",
processNestedResultMappings(context, Collections.emptyList(), resultType));
...
}
外部查詢的最終結果是由 ResultMapping.nestedQueryId 儲存的,之所以這裡強調這些是因為在 ResultSetHandler 解析的時候是分了內外部巢狀兩種大的情況的;
2. 多結果集
此外分析之前首先還要知道 CallableStatement 呼叫儲存過程的時候,會有多結果集的情況,例如:
create procedure sptest.getnamesanditemsbyid(in nameId integer)
modifies sql data
dynamic result sets 2
BEGIN ATOMIC
declare cur1 cursor for select * from sptest.names where id = nameId;
declare cur2 cursor for select * from sptest.items where name_id in (select id from sptest.names where id = nameId);
open cur1;
open cur2;
END
<resultMap type="org.apache.ibatis.submitted.sptests.Name" id="nameResultLinkedNoMatchingInfo">
<result column="ID" property="id"/>
<result column="FIRST_NAME" property="firstName"/>
<result column="LAST_NAME" property="lastName"/>
<collection property="items" resultSet="items" resultMap="itemResult"/>
</resultMap>
<select id="getNamesAndItemsLinkedById" statementType="CALLABLE" resultSets="names,items" resultMap="nameResultLinkedNoMatchingInfo">
{call sptest.getnamesanditemsbyid(#{id,jdbcType=INTEGER,mode=IN})}
</select>
2. 整體流程
上圖就是 ResultSetHandler.handleResultSet 的主要流程,這裡只保留了重要的部分:
- 內外部巢狀查詢的分支;
- 外部巢狀查詢與一級快取;
- 外部巢狀查詢的延遲載入,主要是代理物件、ResultLoader、ResultLoaderMap三個物件;
其餘的部分這裡就不再詳細分析了,一下將主要講解外部巢狀查詢的延遲載入;
二、cglib 和 javassisit 動態代理
在講解延遲載入之前,需要首先簡單瞭解一下動態代理,因為普通的 JavaBean 物件一般都沒有實現介面,所以不能使用 java.lang.reflect.Proxy,在 mybatis 中提供了另外兩種動態代理 cglib 和 javassisit;
1. cglib
public class Car {
String name;
public String getName() { return name; }
public void setName(String name) { this.name = name; }
}
@Test
public void test() {
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(Car.class);
enhancer.setCallback((MethodInterceptor) (obj, method, args, proxy) -> {
System.out.println("代理開始");
Object object = proxy.invokeSuper(obj, args);
System.out.println("result: " + object);
System.out.println("代理結束");
return object;
});
Car car = (Car) enhancer.create();
car.setName("Test");
car.getName();
}
列印:
代理開始
result: null
代理結束
代理開始
result: Test
代理結束
2. javassist
public class Car {
String name;
public String getName() { return name; }
public void setName(String name) { this.name = name; }
}
@Test
public void test() throws IllegalAccessException, InstantiationException {
ProxyFactory proxyFactory = new ProxyFactory();
proxyFactory.setSuperclass(Car.class);
// 設定攔截目標
proxyFactory.setFilter(m -> m.getName().startsWith("get") || m.getName().startsWith("set"));
proxyFactory.setHandler((self, thisMethod, proceed, arg) -> {
System.out.println("代理開始");
Object object = proceed.invoke(self, arg);
System.out.println("result: " + object);
System.out.println("代理結束");
return object;
});
Class clazz = proxyFactory.createClass();
Car car = (Car) clazz.newInstance();
car.setName("Test");
car.getName();
}
列印:
代理開始
result: null
代理結束
代理開始
result: Test
代理結束
三、延遲載入
通過上面的講解大家應該清楚只有外部巢狀查詢才有延遲載入功能;此外和延遲載入相關的配置:
- proxyFactory:(CGLIB | JAVASSIST-預設)指定 mybatis 延遲載入的代理工具;
- lazyLoadingEnabled:(true | false-預設)延遲載入的全域性開關。可使用 association、collection 的 fetchType (lazy|eager)屬性覆蓋;
- aggressiveLazyLoading:(false| true-預設)當開啟時,任何方法的呼叫都會載入該物件的所有屬性。 否則每個屬性會按需載入;
1. demo
<setting name="proxyFactory" value="JAVASSIST"/>
<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>
<resultMap id="user" type="org.apache.ibatis.submitted.lazy_properties.User">
<id property="id" column="id"/>
<result property="name" column="name"/>
</resultMap>
<resultMap id="userWithLazyProperties" type="org.apache.ibatis.submitted.lazy_properties.User" extends="user">
<association property="lazy1" column="id" select="getLazy1" fetchType="lazy"/>
<association property="lazy2" column="id" select="getLazy2" fetchType="eager"/>
<collection property="lazy3" column="id" select="getLazy3" fetchType="lazy"/>
</resultMap>
<select id="getUser" resultMap="userWithLazyProperties">
select * from users where id = #{id}
</select>
public class User {
private Integer id;
private String name;
private User lazy1;
private User lazy2;
private List<User> lazy3;
...
}
@Test
void test() {
try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
Mapper mapper = sqlSession.getMapper(Mapper.class);
User user = mapper.getUser(1);
System.out.println("----getLazy1: " + user.getLazy1());
System.out.println("----getLazy2: " + user.getLazy2());
System.out.println("----getLazy3: " + user.getLazy3());
}
}
列印:
DEBUG [main] - ==> Preparing: select * from users where id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - ====> Preparing: select 12 id, 'lazy2' name from (values(0))
DEBUG [main] - ====> Parameters:
DEBUG [main] - <==== Total: 1
DEBUG [main] - <== Total: 1
DEBUG [main] - ==> Preparing: select 11 id, 'lazy1' name from (values(0))
DEBUG [main] - ==> Parameters:
DEBUG [main] - <== Total: 1
----getLazy1: User{id=11, name='lazy1'}
----getLazy2: User{id=12, name='lazy2'}
DEBUG [main] - ==> Preparing: select 13 id, 'lazy3' name from (values(0))
DEBUG [main] - ==> Parameters:
DEBUG [main] - <== Total: 1
----getLazy3: [User{id=13, name='lazy3'}]
從列印的順序可以看出當 mapper.getUser(1)
的時候,就已經獲取了 user 和 lazy2,而 lazy1 和 lazy3 則是在 get 的時候才載入;這裡在看一下 aggressiveLazyLoading = true 的效果:
列印:
DEBUG [main] - ==> Preparing: select * from users where id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - ====> Preparing: select 12 id, 'lazy2' name from (values(0))
DEBUG [main] - ====> Parameters:
DEBUG [main] - <==== Total: 1
DEBUG [main] - ====> Preparing: select 11 id, 'lazy1' name from (values(0))
DEBUG [main] - ====> Parameters:
DEBUG [main] - <==== Total: 1
DEBUG [main] - ====> Preparing: select 13 id, 'lazy3' name from (values(0))
DEBUG [main] - ====> Parameters:
DEBUG [main] - <==== Total: 1
DEBUG [main] - <== Total: 1
----getLazy1: User{id=11, name='lazy1'}
----getLazy2: User{id=12, name='lazy2'}
----getLazy3: [User{id=13, name='lazy3'}]
這裡也能看到首先是獲取 user 和 lazy2,然後在 user.getLazy1()
的時候同時載入了 lazy1 和 lazy3;
2. 建立代理
在上面已經講過了,在使用延遲載入的時候:
- 首先判斷是否有延遲載入屬性,有就使用代理包裝結果集物件;
- 然後判斷一級快取中時候有對應的外部巢狀,有就取快取;如果沒有就將外部巢狀包裝為 ResultLoader 物件;
- 然後判斷外部巢狀是否需要延遲載入,如果是就將 ResultLoader 加入到 ResultLoaderMap 中,如果不需要就直接載入 resultLoader.loadResult();
建立代理:首先獲取代理工廠,然後建立代理類;
private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, ResultLoaderMap lazyLoader, String columnPrefix) throws SQLException {
this.useConstructorMappings = false; // reset previous mapping result
final List<Class<?>> constructorArgTypes = new ArrayList<>();
final List<Object> constructorArgs = new ArrayList<>();
Object resultObject = createResultObject(rsw, resultMap, constructorArgTypes, constructorArgs, columnPrefix);
if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
for (ResultMapping propertyMapping : propertyMappings) {
if (propertyMapping.getNestedQueryId() != null && propertyMapping.isLazy()) {
resultObject = configuration.getProxyFactory().createProxy(resultObject, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
break;
}
}
}
this.useConstructorMappings = resultObject != null && !constructorArgTypes.isEmpty();
return resultObject;
}
3. 代理工廠
這裡 CglibProxyFactory 和 JavassistProxyFactory 的流程都是一樣的,所以我們就以 CglibProxyFactory 為例進行簡單分析:
crateProxy:
static Object crateProxy(Class<?> type, Callback callback, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
Enhancer enhancer = new Enhancer();
enhancer.setCallback(callback);
enhancer.setSuperclass(type);
try {
type.getDeclaredMethod(WRITE_REPLACE_METHOD);
// ObjectOutputStream will call writeReplace of objects returned by writeReplace
if (LogHolder.log.isDebugEnabled()) {
LogHolder.log.debug(WRITE_REPLACE_METHOD + " method was found on bean " + type + ", make sure it returns this");
}
} catch (NoSuchMethodException e) {
enhancer.setInterfaces(new Class[]{WriteReplaceInterface.class});
} catch (SecurityException e) {
// nothing to do here
}
Object enhanced;
if (constructorArgTypes.isEmpty()) {
enhanced = enhancer.create();
} else {
Class<?>[] typesArray = constructorArgTypes.toArray(new Class[constructorArgTypes.size()]);
Object[] valuesArray = constructorArgs.toArray(new Object[constructorArgs.size()]);
enhanced = enhancer.create(typesArray, valuesArray);
}
return enhanced;
}
這裡建立大致和上面給出的 demo 差不多,都是指定父類,設定回撥;接下來我們繼續看攔截的具體內容:
private final ResultLoaderMap lazyLoader;
public Object intercept(Object enhanced, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
final String methodName = method.getName();
try {
// 鎖定 ResultLoaderMap 物件
synchronized (lazyLoader) {
// 建立代理的時候設定的 WriteReplaceInterface 介面
if (WRITE_REPLACE_METHOD.equals(methodName)) {
Object original;
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 CglibSerialStateHolder(original, lazyLoader.getProperties(), objectFactory, constructorArgTypes, constructorArgs);
} else {
return original;
}
// 真正延遲載入的邏輯處理
} else {
// ResultLoaderMap 數量大於 0,就表示還有待載入的屬性
if (lazyLoader.size() > 0 && !FINALIZE_METHOD.equals(methodName)) {
// aggressive = true,或者 equals、clone、hashCode、toString 之一,就載入全部方法
if (aggressive || lazyLoadTriggerMethods.contains(methodName)) {
lazyLoader.loadAll();
// 呼叫某屬性的 set 方法時,表示不需要從資料庫再載入了,所以將其移除
} else if (PropertyNamer.isSetter(methodName)) {
final String property = PropertyNamer.methodToProperty(methodName);
lazyLoader.remove(property);
// 載入單個屬性
} else if (PropertyNamer.isGetter(methodName)) {
final String property = PropertyNamer.methodToProperty(methodName);
if (lazyLoader.hasLoader(property)) {
lazyLoader.load(property);
}
}
}
}
}
return methodProxy.invokeSuper(enhanced, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
4. 延遲載入屬性載入
// org.apache.ibatis.executor.loader.ResultLoaderMap
public boolean load(String property) throws SQLException {
// 先刪除 key,防止第二次查詢資料庫
LoadPair pair = loaderMap.remove(property.toUpperCase(Locale.ENGLISH));
if (pair != null) {
// 查資料庫
pair.load();
return true;
}
return false;
}
public void load(final Object userObject) throws SQLException {
if (this.metaResultObject == null || this.resultLoader == null) {
...
this.metaResultObject = config.newMetaObject(userObject);
this.resultLoader = new ResultLoader(config, new ClosedExecutor(), ms, this.mappedParameter,
metaResultObject.getSetterType(this.property), null, null);
}
/* We are using a new executor because we may be (and likely are) on a new thread
* and executors aren't thread safe. (Is this sufficient?)
*
* A better approach would be making executors thread safe. */
if (this.serializationCheck == null) {
final ResultLoader old = this.resultLoader;
this.resultLoader = new ResultLoader(old.configuration, new ClosedExecutor(), old.mappedStatement,
old.parameterObject, old.targetType, old.cacheKey, old.boundSql);
}
// 查詢資料庫,並反射設定屬性
this.metaResultObject.setValue(property, this.resultLoader.loadResult());
}
// org.apache.ibatis.executor.loader.ResultLoader
public Object loadResult() throws SQLException {
// 查詢結果
List<Object> list = selectList();
// 轉換結果型別
resultObject = resultExtractor.extractObjectFromList(list, targetType);
return resultObject;
}
// 這裡又是從 Executor 出發,再查資料庫了
private <E> List<E> selectList() throws SQLException {
Executor localExecutor = executor;
if (Thread.currentThread().getId() != this.creatorThreadId || localExecutor.isClosed()) {
localExecutor = newExecutor();
}
try {
return localExecutor.<E> query(mappedStatement, parameterObject, RowBounds.DEFAULT, Executor.NO_RESULT_HANDLER, cacheKey, boundSql);
} finally {
if (localExecutor != executor) {
localExecutor.close(false);
}
}
}
以上就是延遲載入的全部流程了,
5. 延遲載入與一級快取
上面我們將了當一級快取中有外部巢狀查詢快取的時候,會直接取快取,而不是延遲載入:
private Object getNestedQueryMappingValue(ResultSet rs, MetaObject metaResultObject, ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix)
throws SQLException {
final String nestedQueryId = propertyMapping.getNestedQueryId();
final String property = propertyMapping.getProperty();
final MappedStatement nestedQuery = configuration.getMappedStatement(nestedQueryId);
final Class<?> nestedQueryParameterType = nestedQuery.getParameterMap().getType();
final Object nestedQueryParameterObject = prepareParameterForNestedQuery(rs, propertyMapping, nestedQueryParameterType, columnPrefix);
Object value = null;
if (nestedQueryParameterObject != null) {
final BoundSql nestedBoundSql = nestedQuery.getBoundSql(nestedQueryParameterObject);
final CacheKey key = executor.createCacheKey(nestedQuery, nestedQueryParameterObject, RowBounds.DEFAULT, nestedBoundSql);
final Class<?> targetType = propertyMapping.getJavaType();
// 判斷一級快取
if (executor.isCached(nestedQuery, key)) {
executor.deferLoad(nestedQuery, metaResultObject, property, key, targetType);
value = DEFERRED;
} else {
final ResultLoader resultLoader = new ResultLoader(configuration, executor, nestedQuery, nestedQueryParameterObject, targetType, key, nestedBoundSql);
if (propertyMapping.isLazy()) {
lazyLoader.addLoader(property, metaResultObject, resultLoader);
value = DEFERRED;
} else {
value = resultLoader.loadResult();
}
}
}
return value;
}
下面我們就實驗一下:
<setting name="proxyFactory" value="JAVASSIST"/>
<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>
<setting name="localCacheScope" value="SESSION"/>
<resultMap id="FatherMap" type="Father">
<id property="id" column="id"/>
<result property="name" column="name"/>
<association property="grandFather" column="grand_father_id"
select="org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper.selectById"
fetchType="lazy"/>
</resultMap>
<select id="selectById" resultMap="FatherMap" parameterType="int">
SELECT id, name, grand_father_id FROM Father WHERE id = #{id}
</select>
<resultMap id="GrandFatherMap" type="GrandFather">
<id property="id" column="id"/>
<result property="name" column="name"/>
</resultMap>
<select id="selectById" resultMap="GrandFatherMap" parameterType="int">
SELECT id, name FROM GrandFather WHERE id = #{id}
</select>
@Test
void test1() {
try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
GrandFatherMapper grandFatherMapper = sqlSession.getMapper(GrandFatherMapper.class);
FatherMapper fatherMapper = sqlSession.getMapper(FatherMapper.class);
GrandFather grandFather = grandFatherMapper.selectById(1);
System.out.println("----- get grandFather: " + grandFather);
Father father = fatherMapper.selectById(1);
System.out.println("----- get father: " + father.getName());
System.out.println("----- get father.grandFather: " + father.getGrandFather());
}
}
列印:
DEBUG [main] - ==> Preparing: SELECT id, name FROM GrandFather WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <== Total: 1
----- get grandFather: GrandFather{id=1, name='John Smith sen'}
DEBUG [main] - ==> Preparing: SELECT id, name, grand_father_id FROM Father WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <== Total: 1
----- get father: John Smith
----- get father.grandFather: GrandFather{id=1, name='John Smith sen'}
這裡我們首先獲取了一次 GrandFather,保證一級快取中有,然後獲取 Father,延遲載入 GrandFather;從上面的結果可以看到,確實延遲載入是從一級快取中取的;
6. 延遲載入與二級快取
上面我們講過了外部巢狀查詢的時候是從 Executor 開始的,那麼必然有一級快取和二級快取;這裡先說結論巢狀查詢使用二級快取一定要在同一個 namespace 裡面,否則會出現髒讀現象;下面舉例說明:
<setting name="proxyFactory" value="JAVASSIST"/>
<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>
<setting name="localCacheScope" value="STATEMENT"/>
<setting name="cacheEnabled" value="true"/>
// org/apache/ibatis/submitted/lazyload_common_property/FatherMapper.xml
<mapper namespace="org.apache.ibatis.submitted.lazyload_common_property.FatherMapper">
<cache/>
<resultMap id="FatherMap" type="Father">
<id property="id" column="id"/>
<result property="name" column="name"/>
<association property="grandFather" column="grand_father_id"
select="org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper.selectById"
fetchType="lazy"/>
</resultMap>
<select id="selectById" resultMap="FatherMap" parameterType="int">
SELECT id, name, grand_father_id FROM Father WHERE id = #{id}
</select>
<update id="updateById" flushCache="true">
update Father set name = #{name} where id = #{id}
</update>
</mapper>
// org/apache/ibatis/submitted/lazyload_common_property/GrandFatherMapper.xml
<mapper namespace="org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper">
<cache/>
<resultMap id="GrandFatherMap" type="GrandFather">
<id property="id" column="id"/>
<result property="name" column="name"/>
</resultMap>
<select id="selectById" resultMap="GrandFatherMap" parameterType="int">
SELECT id, name FROM GrandFather WHERE id = #{id}
</select>
<update id="updateById" flushCache="true">
update GrandFather set name = #{name} where id = #{id}
</update>
</mapper>
@Test
void test2() {
try (SqlSession sqlSession1 = sqlSessionFactory.openSession();
SqlSession sqlSession2 = sqlSessionFactory.openSession();
) {
GrandFatherMapper grandFatherMapper1 = sqlSession1.getMapper(GrandFatherMapper.class);
GrandFatherMapper grandFatherMapper2 = sqlSession2.getMapper(GrandFatherMapper.class);
FatherMapper fatherMapper1 = sqlSession1.getMapper(FatherMapper.class);
FatherMapper fatherMapper2 = sqlSession2.getMapper(FatherMapper.class);
Father father1 = fatherMapper1.selectById(1);
System.out.println("----- session1 get father(put cache): " + father1);
sqlSession1.commit();
Father father2 = fatherMapper2.selectById(1);
System.out.println("----- session2 get father(get cache): " + father2);
// 測試重點
// fatherMapper1.updateById(1, "TestName");
grandFatherMapper1.updateById(1, "TestName");
sqlSession1.commit();
System.out.println("----- session1 update(put cache)");
Father father3 = fatherMapper2.selectById(1);
System.out.println("----- session2 get father(get cache): " + father3);
}
}
測試流程:
- 首先 session1 查詢並提交二級快取
- 然後 session2 查詢檢查二級快取是否生效
- 然後 session1 修改快取,並提交
- 最後 session2 再查查詢,看是否檢查到快取的修改
列印:
DEBUG [main] - ==> Preparing: SELECT id, name, grand_father_id FROM Father WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <== Total: 1
DEBUG [main] - Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper]: 0.0
DEBUG [main] - ==> Preparing: SELECT id, name FROM GrandFather WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <== Total: 1
----- session1 get father(put cache): Father{id=1, name='John Smith', grandFather=GrandFather{id=1, name='John Smith sen'}}
DEBUG [main] - Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.FatherMapper]: 0.5
----- session2 get father(get cache): Father{id=1, name='John Smith', grandFather=GrandFather{id=1, name='John Smith sen'}}
DEBUG [main] - ==> Preparing: update GrandFather set name = ? where id = ?
DEBUG [main] - ==> Parameters: TestName(String), 1(Integer)
DEBUG [main] - <== Updates: 1
DEBUG [main] - Committing JDBC Connection [org.hsqldb.jdbc.JDBCConnection@2f01783a]
----- session1 update(put cache)
DEBUG [main] - Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.FatherMapper]: 0.6666666666666666
----- session2 get father(get cache): Father{id=1, name='John Smith', grandFather=GrandFather{id=1, name='John Smith sen'}}
注意看這裡二級快取生效了,但是出現了髒讀:
然後我們將上面的註釋開啟:
DEBUG [main] - ==> Preparing: SELECT id, name, grand_father_id FROM Father WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <== Total: 1
DEBUG [main] - Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper]: 0.0
DEBUG [main] - ==> Preparing: SELECT id, name FROM GrandFather WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <== Total: 1
----- session1 get father(put cache): Father{id=1, name='John Smith', grandFather=GrandFather{id=1, name='John Smith sen'}}
DEBUG [main] - Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.FatherMapper]: 0.5
----- session2 get father(get cache): Father{id=1, name='John Smith', grandFather=GrandFather{id=1, name='John Smith sen'}}
DEBUG [main] - ==> Preparing: update Father set name = ? where id = ?
DEBUG [main] - ==> Parameters: TestName(String), 1(Integer)
DEBUG [main] - <== Updates: 1
DEBUG [main] - ==> Preparing: update GrandFather set name = ? where id = ?
DEBUG [main] - ==> Parameters: TestName(String), 1(Integer)
DEBUG [main] - <== Updates: 1
DEBUG [main] - Committing JDBC Connection [org.hsqldb.jdbc.JDBCConnection@2f01783a]
----- session1 update(put cache)
DEBUG [main] - Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.FatherMapper]: 0.3333333333333333
DEBUG [main] - ==> Preparing: SELECT id, name, grand_father_id FROM Father WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <== Total: 1
DEBUG [main] - Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper]: 0.0
DEBUG [main] - ==> Preparing: SELECT id, name FROM GrandFather WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <== Total: 1
----- session2 get father(get cache): Father{id=1, name='TestName', grandFather=GrandFather{id=1, name='TestName'}}
這次發現髒讀消失了??其原因就是第一次之修改了 GrandFather,雖然 Father 中有 GrandFather 屬性,但是重新整理快取的時候並不會重新整理 Father,所以出現的髒讀;其解決辦法就是使用
這裡再提醒一下本文中使用的測試案例都能在 mybatis 原始碼的單元測試用找到