Mybatis二级缓存源码详解（一）

绝地灬酷狼 2022-05-26 10:18 214阅读 0赞

解析基于Mybatis 3.4.4 Release，近几个版本应该都适用。

Mybatis二级缓存总开关默认是开启的。

public class Configuration {
        ......
        protected boolean cacheEnabled = true;
    }

这样SqlSessionFactory在生成SqlSession时，就会把原来的Eexcutor包装成一个CachingExecutor：

public class DefaultSqlSessionFactory implements SqlSessionFactory { 
    
        private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
            Transaction tx = null;
            try {
              final Environment environment = configuration.getEnvironment();
              final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
              tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
              // 生成Executor
              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();
            }
        }
    
    }
    
    
    public class Configuration { 
    
        protected boolean cacheEnabled = true;
    
        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);
        }
        if (cacheEnabled) {   // 包装成 CachingExecutor
          executor = new CachingExecutor(executor);
        }
        executor = (Executor) interceptorChain.pluginAll(executor);
        return executor;
      }
    
    }

下面我们看看CachingExecutor在查询时是如何使用缓存的：

// CachingExecutor 实现Executor 接口,同时又持有一个Executor 的实例，大多数方法最终还是要转发到delegate去执行
    // CachingExecutor 对 delegate 做了装饰，或者在查询时做了静态代理，如果有缓存则直接返回，没有就用delegate去执行
    public class CachingExecutor implements Executor { 
    
      private Executor delegate;
      private TransactionalCacheManager tcm = new TransactionalCacheManager();
    
      @Override
      public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
          throws SQLException {
        Cache cache = ms.getCache();   // 获取MappedStatement 上的缓存对象
        if (cache != null) {
          flushCacheIfRequired(ms);
          if (ms.isUseCache() && resultHandler == null) {
            ensureNoOutParams(ms, parameterObject, boundSql);
            // 通过CacheKey 作为key去缓存里搜索
            List<E> list = (List<E>) tcm.getObject(cache, key);   
            if (list == null) {      
              // 如果缓存value不存在，那么借助delegate执行查询，然后存入Cache,这样相同的查询下次就能复用此结果了 
              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);
      }
    
      // 是否要刷新缓存
      private void flushCacheIfRequired(MappedStatement ms) {
        Cache cache = ms.getCache();
        if (cache != null && ms.isFlushCacheRequired()) {      
          tcm.clear(cache);
        }
      }
    
    }

以上代码有两点关键：

1.  Cache cache = MappedStatement .getCache();
2.  tcm.getObject(cache, key); tcm.putObject(cache, key, list);

我们先看第一点吧，解析Cache对象的实例化过程。

Mybatis在构建SqlSessionFactory时，会解析指定路径下的所有Mapper.xml文件：

public class XMLMapperBuilder extends BaseBuilder { 
    
        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"));
                cacheElement(context.evalNode("cache"));              //解析cache节点
                parameterMapElement(context.evalNodes("/mapper/parameterMap"));
                resultMapElements(context.evalNodes("/mapper/resultMap"));
                sqlElement(context.evalNodes("/mapper/sql"));
                buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
            } catch (Exception e) {
                throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
            }
        }
    
    }

以下就是一个< cache>节点的定义形式：

下面看< cache>节点的解析代码：

public class XMLMapperBuilder extends BaseBuilder { 
    
        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);
                Properties props = context.getChildrenAsProperties();
                builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
            }
        }
    
    }

在解析< cache>节点时，其元素大部分都有默认值，通过typeAliasRegistry将值转换为想要的类型：

public class Configuration {
    
        public Configuration() {
            ......
            typeAliasRegistry.registerAlias("PERPETUAL", PerpetualCache.class);
            typeAliasRegistry.registerAlias("FIFO", FifoCache.class);
            typeAliasRegistry.registerAlias("LRU", LruCache.class);
            typeAliasRegistry.registerAlias("SOFT", SoftCache.class);
            typeAliasRegistry.registerAlias("WEAK", WeakCache.class);
            ......
        }
    
    }

Configuration 在实例化时就为typeAliasRegistry注册了很多别名，在解析Mapper.xml时就能通过别名来获取到真实的Class类型。

接下来看Cache对象的创建过程：

public class MapperBuilderAssistant extends BaseBuilder { 
    
        private String currentNamespace;
        private Cache currentCache;
    
        public Cache useNewCache(Class<? extends Cache> typeClass, Class<? extends Cache> evictionClass, Long flushInterval, Integer size, boolean readWrite, boolean blocking, Properties props) { 
        // 每个Cache都是和currentNamespace紧密关联的，也就是Cache和Mapper.xml里面的namespace元素关联的，也就是每个Mapper匹配一个Cache
        // 也就是说Mapper.xml内元素解析成成的MappedStatement共用一个Cache对象
        Cache cache = new CacheBuilder(currentNamespace)  
            .implementation(valueOrDefault(typeClass, PerpetualCache.class))
            .addDecorator(valueOrDefault(evictionClass, LruCache.class))
            .clearInterval(flushInterval)
            .size(size)
            .readWrite(readWrite)
            .blocking(blocking)
            .properties(props)
            .build();                    // 开始通过建造者构建Cache
        configuration.addCache(cache);   // 将Cache对象添加到Configuration中
        currentCache = cache;            //设置currentCache = cache
        return cache;
      }
    
    }

已经将所有Cache所有相关的数据注入到CacheBuilder中了，接下来看其build( )方法：

public class CacheBuilder {
        private String id;
        private Class<? extends Cache> implementation;
        private List<Class<? extends Cache>> decorators;
        private Integer size;
        private Long clearInterval;
        private boolean readWrite;
        private Properties properties;
        private boolean blocking;
    
        public Cache build() {
            setDefaultImplementations();      
            Cache cache = newBaseCacheInstance(implementation, id);
            setCacheProperties(cache);
            // issue #352, do not apply decorators to custom caches
            if (PerpetualCache.class.equals(cache.getClass())) {
                for (Class<? extends Cache> decorator : decorators) {
                    // 将PerpetualCache外层装饰上 < cache> 内 "eviction" 指定的缓存淘汰策略,可以是 "FIFO","LRU","SOFT","WEAK" 中的一个
                    cache = newCacheDecoratorInstance(decorator, cache);
                    setCacheProperties(cache);
                }
                // 根据< cache> 的数据，在Cache对象外层再依次封装上相应的装饰类
                cache = setStandardDecorators(cache);
            } else if (!LoggingCache.class.isAssignableFrom(cache.getClass())) {
                cache = new LoggingCache(cache);
            }
            return cache;
        }
    
        // Cache的原始实现和原始装饰为空时使用默认值
        private void setDefaultImplementations() {
            if (implementation == null) {
                implementation = PerpetualCache.class;
                if (decorators.isEmpty()) {
                    decorators.add(LruCache.class);
                }
            }
        }
    
        // 根据< cache> 元素内的数据，生成相应的装饰类,封装原始的Cache对象
        private Cache setStandardDecorators(Cache cache) {
            try {
                MetaObject metaCache = SystemMetaObject.forObject(cache);
                if (size != null && metaCache.hasSetter("size")) {
                    metaCache.setValue("size", size);
                }
                if (clearInterval != null) {   // 加上定时清空策略, 时间单位时小时
                    cache = new ScheduledCache(cache);
                    ((ScheduledCache) cache).setClearInterval(clearInterval);
                }
                if (readWrite) {   // 加上读写装饰，以二进制数组的形式缓存查询结果
                    cache = new SerializedCache(cache);
                }
                cache = new LoggingCache(cache);    // 加上日志装饰,以便打印日志
                cache = new SynchronizedCache(cache);   // 加上同步装饰, 将非线程安全的方法都用 "synchronized" 修饰,确保线程安全
                if (blocking) {
                    cache = new BlockingCache(cache);   // 加上对同一key的阻塞访问装饰
                }
                return cache;
            } catch (Exception e) {
                throw new CacheException("Error building standard cache decorators. Cause: " + e, e);
            }
      }
    
    }

以SynchronizedCache为例，看看其对原始Cache的装饰方式：

public class SynchronizedCache implements Cache { 
    
      private Cache delegate;
    
      public SynchronizedCache(Cache delegate) {
        this.delegate = delegate;
      }
    
      @Override
      public String getId() {
        return delegate.getId();
      }
    
      @Override
      public synchronized int getSize() {
        return delegate.getSize();
      }
    
      @Override
      public synchronized void putObject(Object key, Object object) {
        delegate.putObject(key, object);
      }
    
      @Override
      public synchronized Object getObject(Object key) {
        return delegate.getObject(key);
      }
    
      @Override
      public synchronized Object removeObject(Object key) {
        return delegate.removeObject(key);
      }
    
      @Override
      public synchronized void clear() {
        delegate.clear();
      }
    
      @Override
      public int hashCode() {
        return delegate.hashCode();
      }
    
      @Override
      public boolean equals(Object obj) {
        return delegate.equals(obj);
      }
    
      @Override
      public ReadWriteLock getReadWriteLock() {
        return null;
      }
    
    }

SynchronizedCache 将所有非线程安全的方法都加上了 “synchronized ”关键字，确保了缓存数据操作的线程安全性。因为这里的缓存大部分仅仅是对缓存容器的操作，不会占用太多资源和时间，所以性能还是可以接受的。

总结：这篇博客主要讲了 < cache> 元素的解析和实例化的过程，**使用装饰者模式，根据< cache>元素内的数据定义，来生成相应的装饰者对象去装饰原始的PerpetualCache对象**。