graph LR a (二级缓存) --> b (一级缓存) -->c (数据库)

 <settings>
   <setting name="cacheEnabled" value="true"/>
</settings>

<cache></cache>

 <select id="findById" resultType="com.wuzx.pojo.User" useCache="true">
    select * from user where id = #{id}
</select>

    // 解析 `<mapper />` 节点
    private void configurationElement(XNode context) {
        try {
            // 获得 namespace 属性
            String namespace = context.getStringAttribute("namespace");
            if (namespace == null || namespace.equals("")) {
                throw new BuilderException("Mapper''s namespace cannot be empty");
            }
            // 设置 namespace 属性
            builderAssistant.setCurrentNamespace(namespace);
            // 解析 <cache-ref /> 节点
            cacheRefElement(context.evalNode("cache-ref"));
            // 解析 <cache /> 节点
            cacheElement(context.evalNode("cache"));
            // 已废弃！老式风格的参数映射。内联参数是首选,这个元素可能在将来被移除，这里不会记录。
            parameterMapElement(context.evalNodes("/mapper/parameterMap"));
            // 解析 <resultMap /> 节点们
            resultMapElements(context.evalNodes("/mapper/resultMap"));
            // 解析 <sql /> 节点们
            sqlElement(context.evalNodes("/mapper/sql"));
            // 解析 <select /> <insert /> <update /> <delete /> 节点们
            // 这里会将生成的Cache包装到对应的MappedStatement
            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);
        }
    }
    
        // 解析 <cache /> 标签
    private void cacheElement(XNode context) throws Exception {
        if (context != null) {
            //解析<cache/>标签的type属性，这里我们可以自定义cache的实现类，比如redisCache，如果没有自定义，这里使用和一级缓存相同的PERPETUAL
            String type = context.getStringAttribute("type", "PERPETUAL");
            Class<? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type);
            // 获得负责过期的 Cache 实现类
            String eviction = context.getStringAttribute("eviction", "LRU");
            Class<? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction);
            // 清空缓存的频率。0 代表不清空
            Long flushInterval = context.getLongAttribute("flushInterval");
            // 缓存容器大小
            Integer size = context.getIntAttribute("size");
            // 是否序列化
            boolean readWrite = !context.getBooleanAttribute("readOnly", false);
            // 是否阻塞
            boolean blocking = context.getBooleanAttribute("blocking", false);
            // 获得 Properties 属性
            Properties props = context.getChildrenAsProperties();
            // 创建 Cache 对象
            builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
        }
    }
    
    
       /**
     * 创建 Cache 对象
     *
     * @param typeClass 负责存储的 Cache 实现类
     * @param evictionClass 负责过期的 Cache 实现类
     * @param flushInterval 清空缓存的频率。0 代表不清空
     * @param size 缓存容器大小
     * @param readWrite 是否序列化
     * @param blocking 是否阻塞
     * @param props Properties 对象
     * @return Cache 对象
     */
    public Cache useNewCache(Class<? extends Cache> typeClass,
                             Class<? extends Cache> evictionClass,
                             Long flushInterval,
                             Integer size,
                             boolean readWrite,
                             boolean blocking,
                             Properties props) {

        // 1.生成Cache对象
        Cache cache = new CacheBuilder(currentNamespace)
                //这里如果我们定义了<cache/>中的type，就使用自定义的Cache,否则使用和一级缓存相同的PerpetualCache
                .implementation(valueOrDefault(typeClass, PerpetualCache.class))
                .addDecorator(valueOrDefault(evictionClass, LruCache.class))
                .clearInterval(flushInterval)
                .size(size)
                .readWrite(readWrite)
                .blocking(blocking)
                .properties(props)
                .build();
        // 2.添加到Configuration中
        configuration.addCache(cache);
        // 3.并将cache赋值给MapperBuilderAssistant.currentCache
        currentCache = cache;
        return cache;
    }

    /**
     * Cache 对象集合
     *
     * KEY：命名空间 namespace
     */
    protected final Map<String, Cache> caches = new StrictMap<>("Caches collection");

    public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
        // 获得 BoundSql 对象
        BoundSql boundSql = ms.getBoundSql(parameterObject);
        // 创建 CacheKey 对象
        CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
        // 查询
        return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
    }

    public Object getObject(Object key) {
        // 查询的时候是直接从delegate中去查询的，也就是从真正的缓存对象中查询
        Object object = delegate.getObject(key);
        // 如果不存在，则添加到 entriesMissedInCache 中
        if (object == null) {
            // 缓存未命中，则将 key 存入到 entriesMissedInCache 中
            entriesMissedInCache.add(key);
        }
        // issue #146
        // 如果 clearOnCommit 为 true ，表示处于持续清空状态，则返回 null
        if (clearOnCommit) {
            return null;
        // 返回 value
        } else {
            return object;
        }
    }

    public void putObject(Object key, Object object) {
        // 将键值对存入到 entriesToAddOnCommit 这个Map中中，而非真实的缓存对象 delegate 中
        entriesToAddOnCommit.put(key, object);
    }	

    public void commit(boolean force) {
        try {
            // 提交事务
            executor.commit(isCommitOrRollbackRequired(force));
            // 标记 dirty 为 false
            dirty = false;
        } catch (Exception e) {
            throw ExceptionFactory.wrapException("Error committing transaction.  Cause: " + e, e);
        } finally {
            ErrorContext.instance().reset();
        }
    }
    public void commit(boolean required) throws SQLException {
        // 执行 delegate 对应的方法
        delegate.commit(required);
        // 提交 TransactionalCacheManager
        tcm.commit();
    }
    /**
     * 提交所有 TransactionalCache
     */
    public void commit() {
        for (TransactionalCache txCache : transactionalCaches.values()) {
            txCache.commit();
        }
    }

    public int update(MappedStatement ms, Object parameterObject) throws SQLException {
        // 如果需要清空缓存，则进行清空
        flushCacheIfRequired(ms);
        // 执行 delegate 对应的方法
        return delegate.update(ms, parameterObject);
    }

    /**
     * 如果需要清空缓存，则进行清空
     *
     * @param ms MappedStatement 对象
     */
    private void flushCacheIfRequired(MappedStatement ms) {
        Cache cache = ms.getCache();
        if (cache != null && ms.isFlushCacheRequired()) { // 是否需要清空缓存
            tcm.clear(cache);
        }
    }

package mybatis;

import mybatis.bean.StudentBean;
import mybatis.dao.StudentMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;

public class Test1 {

    public SqlSession session;
    public SqlSessionFactory sqlSessionFactory;

    @Before
    public void init() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        session =  sqlSessionFactory.openSession();
    }

    @Test
    public void studentTest(){
        StudentMapper mapper = session.getMapper(StudentMapper.class);
        StudentBean result = mapper.selectUser(1);//这句执行了sql,也就是说,这句给一级缓存塞了值
        StudentBean result2 = mapper.selectUser(1);//这句执行了sql,也就是说,这句给一级缓存塞了值

        System.out.println(result==result2);
    }
}

list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;

package mybatis;

import mybatis.bean.StudentBean;
import mybatis.dao.StudentMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;
import java.io.IOException;
import java.io.InputStream;

public class Test1 {

    public SqlSession session;
    public SqlSessionFactory sqlSessionFactory;

    @Before
    public void init() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        session =  sqlSessionFactory.openSession();
    }

    @Test
    public void studentTest(){
        StudentMapper mapper = session.getMapper(StudentMapper.class);
        StudentBean result = mapper.selectUser(1);
        session.close();

        session = sqlSessionFactory.openSession();
        StudentMapper mapper2 = session.getMapper(StudentMapper.class);
        StudentBean result2 = mapper2.selectUser(1);
        System.out.println(result == result2);

    }

}

<?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="mybatis.dao.StudentMapper">
    <cache></cache>
    <select id="selectUser" resultType="mybatis.bean.StudentBean">
        select * from student t where t.id = #{id}
    </select>
</mapper>

package mybatis.dao;

import mybatis.bean.StudentBean;
import org.apache.ibatis.annotations.CacheNamespace;
import org.apache.ibatis.annotations.Select;


@CacheNamespace
public interface StudentMapper {

    @Select("select * from student t where t.id = #{id}")
    StudentBean selectUser(int id);
}

@CacheNamespace(readWrite = false)

package mybatis;

import mybatis.bean.StudentBean;
import mybatis.dao.StudentMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;
import java.io.IOException;
import java.io.InputStream;

public class Test1 {

    public SqlSession session;
    public SqlSessionFactory sqlSessionFactory;

    @Before
    public void init() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        session =  sqlSessionFactory.openSession();
    }

    @Test
    public void studentTest(){
        StudentMapper mapper = session.getMapper(StudentMapper.class);
        StudentBean result = mapper.selectUser(1);
        System.out.println(result);
        result.setId(2222);
        session.commit();

        session = sqlSessionFactory.openSession();
        StudentMapper mapper2 = session.getMapper(StudentMapper.class);
        StudentBean result2 = mapper2.selectUser(1);
        System.out.println(result2);

        System.out.println(result == result2);
    }
}

二级缓存

SELECT * FROM user where id > ?

public class CacheKey implements Cloneable, Serializable {

    private static final int DEFAULT_MULTIPLYER = 37;
    private static final int DEFAULT_HASHCODE = 17;

    // 乘子，默认为37
    private final int multiplier;
    // CacheKey 的 hashCode，综合了各种影响因子
    private int hashcode;
    // 校验和
    private long checksum;
    // 影响因子个数
    private int count;
    // 影响因子集合
    private List<Object> updateList;
    
    public CacheKey() {
        this.hashcode = DEFAULT_HASHCODE;
        this.multiplier = DEFAULT_MULTIPLYER;
        this.count = 0;
        this.updateList = new ArrayList<Object>();
    }
    
    /** 每当执行更新操作时，表示有新的影响因子参与计算 
     *  当不断有新的影响因子参与计算时，hashcode 和 checksum 将会变得愈发复杂和随机。这样可降低冲突率，使 CacheKey 可在缓存中更均匀的分布。
     */
    public void update(Object object) {
            int baseHashCode = object == null ? 1 : ArrayUtil.hashCode(object);
        // 自增 count
        count++;
        // 计算校验和
        checksum += baseHashCode;
        // 更新 baseHashCode
        baseHashCode *= count;

        // 计算 hashCode
        hashcode = multiplier * hashcode + baseHashCode;

        // 保存影响因子
        updateList.add(object);
    }
    
    /**
     *  CacheKey 最终要作为键存入 HashMap，因此它需要覆盖 equals 和 hashCode 方法
     */
    public boolean equals(Object object) {
        // 检测是否为同一个对象
        if (this == object) {
            return true;
        }
        // 检测 object 是否为 CacheKey
        if (!(object instanceof CacheKey)) {
            return false;
        }
        final CacheKey cacheKey = (CacheKey) object;

        // 检测 hashCode 是否相等
        if (hashcode != cacheKey.hashcode) {
            return false;
        }
        // 检测校验和是否相同
        if (checksum != cacheKey.checksum) {
            return false;
        }
        // 检测 coutn 是否相同
        if (count != cacheKey.count) {
            return false;
        }

        // 如果上面的检测都通过了，下面分别对每个影响因子进行比较
        for (int i = 0; i < updateList.size(); i++) {
            Object thisObject = updateList.get(i);
            Object thatObject = cacheKey.updateList.get(i);
            if (!ArrayUtil.equals(thisObject, thatObject)) {
                return false;
            }
        }
        return true;
    }

    public int hashCode() {
        // 返回 hashcode 变量
        return hashcode;
    }
}

public static void main(String[] args) {
    SqlSession session = sqlSessionFactory.openSession();
    try {
        Blog blog = (Blog)session.selectOne("queryById",1);
        Blog blog2 = (Blog)session.selectOne("queryById",1);
    } finally {
        session.close();
    }
}

public static void main(String[] args) {
    SqlSession session = sqlSessionFactory.openSession();
    SqlSession session1 = sqlSessionFactory.openSession();
    try {
        Blog blog = (Blog)session.selectOne("queryById",17);
        Blog blog2 = (Blog)session1.selectOne("queryById",17);
    } finally {
        session.close();
    }
}

public static void main(String[] args) {
    SqlSession session = sqlSessionFactory.openSession();
    try {
        Blog blog = (Blog)session.selectOne("queryById",17);
        blog.setName("llll");
        session.update("updateBlog",blog);
        
        Blog blog2 = (Blog)session.selectOne("queryById",17);
    } finally {
        session.close();
    }
}

SqlSession session = sessionFactory.openSession();

public SqlSession openSession() {
    return this.openSessionFromDataSource(this.configuration.getDefaultExecutorType(), (TransactionIsolationLevel)null, false);
}

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;

    DefaultSqlSession var8;
    try {
        Environment environment = this.configuration.getEnvironment();
        TransactionFactory transactionFactory = this.getTransactionFactoryFromEnvironment(environment);
        tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
        //创建SQL执行器
        Executor executor = this.configuration.newExecutor(tx, execType);
        var8 = new DefaultSqlSession(this.configuration, executor, autoCommit);
    } catch (Exception var12) {
        this.closeTransaction(tx);
        throw ExceptionFactory.wrapException("Error opening session.  Cause: " + var12, var12);
    } finally {
        ErrorContext.instance().reset();
    }

    return var8;
}

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? this.defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Object executor;
    if (ExecutorType.BATCH == executorType) {
        executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
        executor = new ReuseExecutor(this, transaction);
    } else {
        //默认创建SimpleExecutor
        executor = new SimpleExecutor(this, transaction);
    }

    if (this.cacheEnabled) {
        //开启二级缓存就会用CachingExecutor装饰SimpleExecutor
        executor = new CachingExecutor((Executor)executor);
    }

    Executor executor = (Executor)this.interceptorChain.pluginAll(executor);
    return executor;
}

public SimpleExecutor(Configuration configuration, Transaction transaction) {
    super(configuration, transaction);
}

protected BaseExecutor(Configuration configuration, Transaction transaction) {
    this.transaction = transaction;
    this.deferredLoads = new ConcurrentLinkedQueue();
    //创建一个缓存对象，PerpetualCache并不是线程安全的
    //但SqlSession和Executor对象在通常情况下只能有一个线程访问，而且访问完成之后马上销毁。也就是session.close();
    this.localCache = new PerpetualCache("LocalCache");
    this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
    this.closed = false;
    this.configuration = configuration;
    this.wrapper = this;
}

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);
    //利用sql和执行的参数生成一个key，如果同一sql不同的执行参数的话，将会生成不同的key
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}

@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
    throws SQLException {
    // 这里是二级缓存的查询，我们暂且不看
    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;
        }
    }
    
    // 直接来到这里
    // 实现为BaseExecutor.query()
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
    if (closed) {
        throw new ExecutorException("Executor was closed.");
    }
    // 创建 CacheKey 对象
    CacheKey cacheKey = new CacheKey();
    // 将 MappedStatement 的 id 作为影响因子进行计算
    cacheKey.update(ms.getId());
    // RowBounds 用于分页查询，下面将它的两个字段作为影响因子进行计算
    cacheKey.update(rowBounds.getOffset());
    cacheKey.update(rowBounds.getLimit());
    // 获取 sql 语句，并进行计算
    cacheKey.update(boundSql.getSql());
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
    for (ParameterMapping parameterMapping : parameterMappings) {
        if (parameterMapping.getMode() != ParameterMode.OUT) {
            // 运行时参数
            Object value;    
            // 当前大段代码用于获取 SQL 中的占位符 #{xxx} 对应的运行时参数，
            // 前文有类似分析，这里忽略了
            String propertyName = parameterMapping.getProperty();
            if (boundSql.hasAdditionalParameter(propertyName)) {
                value = boundSql.getAdditionalParameter(propertyName);
            } else if (parameterObject == null) {
                value = null;
            } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
                value = parameterObject;
            } else {
                MetaObject metaObject = configuration.newMetaObject(parameterObject);
                value = metaObject.getValue(propertyName);
            }
            
            // 让运行时参数参与计算
            cacheKey.update(value);
        }
    }
    if (configuration.getEnvironment() != null) {
        // 获取 Environment id 遍历，并让其参与计算
        cacheKey.update(configuration.getEnvironment().getId());
    }
    return cacheKey;
}

@SuppressWarnings("unchecked")
@Override
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.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
        clearLocalCache();
    }
    List<E> list;
    try {
        queryStack++;
        // 看这里，先从localCache中获取对应CacheKey的结果值
        list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
        if (list != null) {
            handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
        } else {
            // 如果缓存中没有值，则从DB中查询
            list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
        }
    } finally {
        queryStack--;
    }
    if (queryStack == 0) {
        for (DeferredLoad deferredLoad : deferredLoads) {
            deferredLoad.load();
        }
        deferredLoads.clear();
        if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
            clearLocalCache();
        }
    }
    return list;
}

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 {
        // 1.执行查询，获取list
        list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
        localCache.removeObject(key);
    }
    // 2.将查询后的结果放置到localCache中，key就是我们刚才封装的CacheKey，value就是从DB中查询到的list
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
        localOutputParameterCache.putObject(key, parameter);
    }
    return list;
}

public class PerpetualCache implements Cache {

    private final String id;

    private Map<Object, Object> cache = new HashMap<Object, Object>();

    public PerpetualCache(String id) {
        this.id = id;
    }

    @Override
    public String getId() {
        return id;
    }

    @Override
    public int getSize() {
        return cache.size();
    }

    @Override
    public void putObject(Object key, Object value) {
        // 存储键值对到 HashMap
        cache.put(key, value);
    }

    @Override
    public Object getObject(Object key) {
        // 查找缓存项
        return cache.get(key);
    }

    @Override
    public Object removeObject(Object key) {
        // 移除缓存项
        return cache.remove(key);
    }

    @Override
    public void clear() {
        cache.clear();
    }
    
    // 省略部分代码
}

@Override
public int update(MappedStatement ms, Object parameter) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
    if (closed) {
        throw new ExecutorException("Executor was closed.");
    }
    //每次执行update/insert/delete语句时都会清除一级缓存。
    clearLocalCache();
    // 然后再进行更新操作
    return doUpdate(ms, parameter);
}
 
@Override
public void clearLocalCache() {
    if (!closed) {
        // 直接将Map清空
        localCache.clear();
        localOutputParameterCache.clear();
    }
}

//DefaultSqlSession
public void close() {
    try {
        this.executor.close(this.isCommitOrRollbackRequired(false));
        this.closeCursors();
        this.dirty = false;
    } finally {
        ErrorContext.instance().reset();
    }

}

//BaseExecutor
public void close(boolean forceRollback) {
    try {
        try {
            this.rollback(forceRollback);
        } finally {
            if (this.transaction != null) {
                this.transaction.close();
            }

        }
    } catch (SQLException var11) {
        log.warn("Unexpected exception on closing transaction.  Cause: " + var11);
    } finally {
        this.transaction = null;
        this.deferredLoads = null;
        this.localCache = null;
        this.localOutputParameterCache = null;
        this.closed = true;
    }

}

public void rollback(boolean required) throws SQLException {
    if (!this.closed) {
        try {
            this.clearLocalCache();
            this.flushStatements(true);
        } finally {
            if (required) {
                this.transaction.rollback();
            }

        }
    }

}

public void clearLocalCache() {
    if (!this.closed) {
        // 直接将Map清空
        this.localCache.clear();
        this.localOutputParameterCache.clear();
    }
}

<?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="mybatis.BlogMapper">
    <select id="queryById" parameterType="int" resultType="jdbc.Blog">
        select * from blog where id = #{id}
    </select>
    <update id="updateBlog" parameterType="jdbc.Blog">
        update Blog set name = #{name},url = #{url} where id=#{id}
    </update>
    <!-- 开启BlogMapper二级缓存 -->
    <cache/>
</mapper>

public static void main(String[] args) {
    SqlSession session = sqlSessionFactory.openSession();
    SqlSession session1 = sqlSessionFactory.openSession();
    try {
        Blog blog = (Blog)session.selectOne("queryById",17);
        Blog blog2 = (Blog)session1.selectOne("queryById",17);
    } finally {
        session.close();
    }
}

public static void main(String[] args) {
    SqlSession session = sqlSessionFactory.openSession();
    SqlSession session1 = sqlSessionFactory.openSession();
    try {
        Blog blog = (Blog)session.selectOne("queryById",17);
        session.commit();
 
        Blog blog2 = (Blog)session1.selectOne("queryById",17);
    } finally {
        session.close();
    }
}

public static void main(String[] args) {
    SqlSession session = sqlSessionFactory.openSession();
    SqlSession session1 = sqlSessionFactory.openSession();
    try {
        Blog blog = (Blog)session.selectOne("queryById",17);
        session.close();
 
        Blog blog2 = (Blog)session1.selectOne("queryById",17);
    } finally {
        session.close();
    }
}

View Code

// XMLMapperBuilder.parse()
public void parse() {
    if (!configuration.isResourceLoaded(resource)) {
        // 解析mapper属性
        configurationElement(parser.evalNode("/mapper"));
        configuration.addLoadedResource(resource);
        bindMapperForNamespace();
    }
 
    parsePendingResultMaps();
    parsePendingChacheRefs();
    parsePendingStatements();
}
 
// configurationElement()
private void configurationElement(XNode context) {
    try {
        String namespace = context.getStringAttribute("namespace");
        if (namespace == null || namespace.equals("")) {
            throw new BuilderException("Mapper''s namespace cannot be empty");
        }
        builderAssistant.setCurrentNamespace(namespace);
        cacheRefElement(context.evalNode("cache-ref"));
        // 最终在这里看到了关于cache属性的处理
        cacheElement(context.evalNode("cache"));
        parameterMapElement(context.evalNodes("/mapper/parameterMap"));
        resultMapElements(context.evalNodes("/mapper/resultMap"));
        sqlElement(context.evalNodes("/mapper/sql"));
        // 这里会将生成的Cache包装到对应的MappedStatement
        buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    } catch (Exception e) {
        throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
    }
}
 
// cacheElement()
private void cacheElement(XNode context) throws Exception {
    if (context != null) {
        //解析<cache/>标签的type属性，这里我们可以自定义cache的实现类，比如redisCache，如果没有自定义，这里使用和一级缓存相同的PERPETUAL
        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);
        Properties props = context.getChildrenAsProperties();
        // 构建Cache对象
        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) {
    // 1.生成Cache对象
    Cache cache = new CacheBuilder(currentNamespace)
         //这里如果我们定义了<cache/>中的type，就使用自定义的Cache,否则使用和一级缓存相同的PerpetualCache
        .implementation(valueOrDefault(typeClass, PerpetualCache.class))
        .addDecorator(valueOrDefault(evictionClass, LruCache.class))
        .clearInterval(flushInterval)
        .size(size)
        .readWrite(readWrite)
        .blocking(blocking)
        .properties(props)
        .build();
    // 2.添加到Configuration中
    configuration.addCache(cache);
    // 3.并将cache赋值给MapperBuilderAssistant.currentCache
    currentCache = cache;
    return cache;
}

// buildStatementFromContext()
private void buildStatementFromContext(List<XNode> list) {
    if (configuration.getDatabaseId() != null) {
        buildStatementFromContext(list, configuration.getDatabaseId());
    }
    buildStatementFromContext(list, null);
}
 
//buildStatementFromContext()
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    for (XNode context : list) {
        final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
        try {
            // 每一条执行语句转换成一个MappedStatement
            statementParser.parseStatementNode();
        } catch (IncompleteElementException e) {
            configuration.addIncompleteStatement(statementParser);
        }
    }
}
 
// XMLStatementBuilder.parseStatementNode();
public void parseStatementNode() {
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");
    ...
 
    Integer fetchSize = context.getIntAttribute("fetchSize");
    Integer timeout = context.getIntAttribute("timeout");
    String parameterMap = context.getStringAttribute("parameterMap");
    String parameterType = context.getStringAttribute("parameterType");
    Class<?> parameterTypeClass = resolveClass(parameterType);
    String resultMap = context.getStringAttribute("resultMap");
    String resultType = context.getStringAttribute("resultType");
    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);
 
    ...
    // 创建MappedStatement对象
    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
                                        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
                                        resultSetTypeEnum, flushCache, useCache, resultOrdered, 
                                        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}
 
// builderAssistant.addMappedStatement()
public MappedStatement addMappedStatement(
    String id,
    ...) {
 
    if (unresolvedCacheRef) {
        throw new IncompleteElementException("Cache-ref not yet resolved");
    }
 
    id = applyCurrentNamespace(id, false);
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    //创建MappedStatement对象
    MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
        ...
        .flushCacheRequired(valueOrDefault(flushCache, !isSelect))
        .useCache(valueOrDefault(useCache, isSelect))
        .cache(currentCache);// 在这里将之前生成的Cache封装到MappedStatement
 
    ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
    if (statementParameterMap != null) {
        statementBuilder.parameterMap(statementParameterMap);
    }
 
    MappedStatement statement = statementBuilder.build();
    configuration.addMappedStatement(statement);
    return statement;
}

// CachingExecutor
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    // 创建 CacheKey
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
    throws SQLException {
    // 从 MappedStatement 中获取 Cache，注意这里的 Cache 是从MappedStatement中获取的
    // 也就是我们上面解析Mapper中<cache/>标签中创建的，它保存在Configration中
    // 我们在上面解析blog.xml时分析过每一个MappedStatement都有一个Cache对象，就是这里
    Cache cache = ms.getCache();
    // 如果配置文件中没有配置 <cache>，则 cache 为空
    if (cache != null) {
        //如果需要刷新缓存的话就刷新：flushCache="true"
        flushCacheIfRequired(ms);
        if (ms.isUseCache() && resultHandler == null) {
            ensureNoOutParams(ms, boundSql);
            // 访问二级缓存
            List<E> list = (List<E>) tcm.getObject(cache, key);
            // 缓存未命中
            if (list == null) {
                // 如果没有值，则执行查询，这个查询实际也是先走一级缓存查询，一级缓存也没有的话，则进行DB查询
                list = delegate.<E>query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
                // 缓存查询结果
                tcm.putObject(cache, key, list);
            }
            return list;
        }
    }
    return delegate.<E>query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

<!-- 执行此语句清空缓存 -->
<select id="getAll" resultType="entity.TDemo" useCache="true" flushCache="true" >
    select * from t_demo
</select>

/** 事务缓存管理器 */
public class TransactionalCacheManager {

    // Cache 与 TransactionalCache 的映射关系表
    private final Map<Cache, TransactionalCache> transactionalCaches = new HashMap<Cache, TransactionalCache>();

    public void clear(Cache cache) {
        // 获取 TransactionalCache 对象，并调用该对象的 clear 方法，下同
        getTransactionalCache(cache).clear();
    }

    public Object getObject(Cache cache, CacheKey key) {
        // 直接从TransactionalCache中获取缓存
        return getTransactionalCache(cache).getObject(key);
    }

    public void putObject(Cache cache, CacheKey key, Object value) {
        // 直接存入TransactionalCache的缓存中
        getTransactionalCache(cache).putObject(key, value);
    }

    public void commit() {
        for (TransactionalCache txCache : transactionalCaches.values()) {
            txCache.commit();
        }
    }

    public void rollback() {
        for (TransactionalCache txCache : transactionalCaches.values()) {
            txCache.rollback();
        }
    }

    private TransactionalCache getTransactionalCache(Cache cache) {
        // 从映射表中获取 TransactionalCache
        TransactionalCache txCache = transactionalCaches.get(cache);
        if (txCache == null) {
            // TransactionalCache 也是一种装饰类，为 Cache 增加事务功能
            // 创建一个新的TransactionalCache，并将真正的Cache对象存进去
            txCache = new TransactionalCache(cache);
            transactionalCaches.put(cache, txCache);
        }
        return txCache;
    }
}

public class TransactionalCache implements Cache {
    //真正的缓存对象，和上面的Map<Cache, TransactionalCache>中的Cache是同一个
    private final Cache delegate;
    private boolean clearOnCommit;
    // 在事务被提交前，所有从数据库中查询的结果将缓存在此集合中
    private final Map<Object, Object> entriesToAddOnCommit;
    // 在事务被提交前，当缓存未命中时，CacheKey 将会被存储在此集合中
    private final Set<Object> entriesMissedInCache;


    @Override
    public Object getObject(Object key) {
        // 查询的时候是直接从delegate中去查询的，也就是从真正的缓存对象中查询
        Object object = delegate.getObject(key);
        if (object == null) {
            // 缓存未命中，则将 key 存入到 entriesMissedInCache 中
            entriesMissedInCache.add(key);
        }

        if (clearOnCommit) {
            return null;
        } else {
            return object;
        }
    }

    @Override
    public void putObject(Object key, Object object) {
        // 将键值对存入到 entriesToAddOnCommit 这个Map中中，而非真实的缓存对象 delegate 中
        entriesToAddOnCommit.put(key, object);
    }

    @Override
    public Object removeObject(Object key) {
        return null;
    }

    @Override
    public void clear() {
        clearOnCommit = true;
        // 清空 entriesToAddOnCommit，但不清空 delegate 缓存
        entriesToAddOnCommit.clear();
    }

    public void commit() {
        // 根据 clearOnCommit 的值决定是否清空 delegate
        if (clearOnCommit) {
            delegate.clear();
        }
        
        // 刷新未缓存的结果到 delegate 缓存中
        flushPendingEntries();
        // 重置 entriesToAddOnCommit 和 entriesMissedInCache
        reset();
    }

    public void rollback() {
        unlockMissedEntries();
        reset();
    }

    private void reset() {
        clearOnCommit = false;
        // 清空集合
        entriesToAddOnCommit.clear();
        entriesMissedInCache.clear();
    }

    private void flushPendingEntries() {
        for (Map.Entry<Object, Object> entry : entriesToAddOnCommit.entrySet()) {
            // 将 entriesToAddOnCommit 中的内容转存到 delegate 中
            delegate.putObject(entry.getKey(), entry.getValue());
        }
        for (Object entry : entriesMissedInCache) {
            if (!entriesToAddOnCommit.containsKey(entry)) {
                // 存入空值
                delegate.putObject(entry, null);
            }
        }
    }

    private void unlockMissedEntries() {
        for (Object entry : entriesMissedInCache) {
            try {
                // 调用 removeObject 进行解锁
                delegate.removeObject(entry);
            } catch (Exception e) {
                log.warn("...");
            }
        }
    }

}

@Override
public void commit(boolean force) {
    try {
        // 主要是这句
        executor.commit(isCommitOrRollbackRequired(force));
        dirty = false;
    } catch (Exception e) {
        throw ExceptionFactory.wrapException("Error committing transaction.  Cause: " + e, e);
    } finally {
        ErrorContext.instance().reset();
    }
}
 
// CachingExecutor.commit()
@Override
public void commit(boolean required) throws SQLException {
    delegate.commit(required);
    tcm.commit();// 在这里
}
 
// TransactionalCacheManager.commit()
public void commit() {
    for (TransactionalCache txCache : transactionalCaches.values()) {
        txCache.commit();// 在这里
    }
}
 
// TransactionalCache.commit()
public void commit() {
    if (clearOnCommit) {
        delegate.clear();
    }
    flushPendingEntries();//这一句
    reset();
}
 
// TransactionalCache.flushPendingEntries()
private void flushPendingEntries() {
    for (Map.Entry<Object, Object> entry : entriesToAddOnCommit.entrySet()) {
        // 在这里真正的将entriesToAddOnCommit的对象逐个添加到delegate中，只有这时，二级缓存才真正的生效
        delegate.putObject(entry.getKey(), entry.getValue());
    }
    for (Object entry : entriesMissedInCache) {
        if (!entriesToAddOnCommit.containsKey(entry)) {
            delegate.putObject(entry, null);
        }
    }
}

public int update(String statement, Object parameter) {
    int var4;
    try {
        this.dirty = true;
        MappedStatement ms = this.configuration.getMappedStatement(statement);
        var4 = this.executor.update(ms, this.wrapCollection(parameter));
    } catch (Exception var8) {
        throw ExceptionFactory.wrapException("Error updating database.  Cause: " + var8, var8);
    } finally {
        ErrorContext.instance().reset();
    }

    return var4;
}

public int update(MappedStatement ms, Object parameterObject) throws SQLException {
    this.flushCacheIfRequired(ms);
    return this.delegate.update(ms, parameterObject);
}

private void flushCacheIfRequired(MappedStatement ms) {
    //获取MappedStatement对应的Cache，进行清空
    Cache cache = ms.getCache();
    //SQL需设置flushCache="true" 才会执行清空
    if (cache != null && ms.isFlushCacheRequired()) {
  this.tcm.clear(cache);
    }
}

<?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="com.tyb.saas.common.dal.dao.AreaDefaultMapper">
 
    <!--
    flushInterval（清空缓存的时间间隔）: 单位毫秒，可以被设置为任意的正整数。
        默认情况是不设置，也就是没有刷新间隔，缓存仅仅调用语句时刷新。
    size（引用数目）: 可以被设置为任意正整数，要记住你缓存的对象数目和你运行环境的可用内存资源数目。默认值是1024。
    readOnly（只读）:属性可以被设置为true或false。只读的缓存会给所有调用者返回缓存对象的相同实例。
        因此这些对象不能被修改。这提供了很重要的性能优势。可读写的缓存会返回缓存对象的拷贝（通过序列化）。这会慢一些，但是安全，因此默认是false。
    eviction（回收策略）: 默认的是 LRU:
        1.LRU – 最近最少使用的:移除最长时间不被使用的对象。
        2.FIFO – 先进先出:按对象进入缓存的顺序来移除它们。
        3.SOFT – 软引用:移除基于垃圾回收器状态和软引用规则的对象。
        4.WEAK – 弱引用:更积极地移除基于垃圾收集器状态和弱引用规则的对象。
    blocking（是否使用阻塞缓存）: 默认为false，当指定为true时将采用BlockingCache进行封装，blocking，阻塞的意思，
        使用BlockingCache会在查询缓存时锁住对应的Key，如果缓存命中了则会释放对应的锁，否则会在查询数据库以后再释放锁，
        这样可以阻止并发情况下多个线程同时查询数据，详情可参考BlockingCache的源码。
    type（缓存类）：可指定使用的缓存类，mybatis默认使用HashMap进行缓存,这里引用第三方中间件进行缓存
    -->
    <cache type="org.mybatis.caches.redis.RedisCache" blocking="false"
           flushInterval="0" readOnly="true" size="1024" eviction="FIFO"/>
 
    <!--
        useCache（是否使用缓存）：默认true使用缓存
    -->
    <select id="find" parameterType="map" resultType="com.chenhao.model.User" useCache="true">
        SELECT * FROM user
    </select>
 
</mapper>

package com.chenhao.mybatis.cache;

import org.apache.ibatis.cache.Cache;
import org.springframework.data.redis.core.RedisTemplate;
import org.springframework.data.redis.core.ValueOperations;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * @author chenhao
 * @date 2019/10/31.
 */
public class RedisCache implements Cache {

    private final String id;

    private static ValueOperations<String, Object> valueOs;

    private static RedisTemplate<String, String> template;


    public static void setValueOs(ValueOperations<String, Object> valueOs) {
        RedisCache.valueOs = valueOs;
    }

    public static void setTemplate(RedisTemplate<String, String> template) {
        RedisCache.template = template;
    }

    private final ReadWriteLock readWriteLock = new ReentrantReadWriteLock();


    public RedisCache(String id) {
        if (id == null) {
            throw new IllegalArgumentException("Cache instances require an ID");
        }
        this.id = id;
    }

    @Override
    public String getId() {
        return this.id;
    }

    @Override
    public void putObject(Object key, Object value) {
        valueOs.set(key.toString(), value, 10, TimeUnit.MINUTES);
    }

    @Override
    public Object getObject(Object key) {
        return valueOs.get(key.toString());
    }

    @Override
    public Object removeObject(Object key) {
        valueOs.set(key.toString(), "", 0, TimeUnit.MINUTES);
        return key;
    }

    @Override
    public void clear() {
        template.getConnectionFactory().getConnection().flushDb();
    }

    @Override
    public int getSize() {
        return template.getConnectionFactory().getConnection().dbSize().intValue();
    }

    @Override
    public ReadWriteLock getReadWriteLock() {
        return this.readWriteLock;
    }
}

<cache type="com.chenhao.mybatis.cache.RedisCache"/>