mybaits 源码分析5 mybatis 底层缓存原理（一级缓存和二级缓存）
首先还是看源码，还是这个DefaultSqlSessionFactory类，看这个方法
@Override
public Connection getConnection() {
try {
//看这里执行这个executor.getTransaction()
return executor.getTransaction().getConnection();
} catch (SQLException e) {
throw ExceptionFactory.wrapException("Error getting a new connection. Cause: " + e, e);
}
}
跟进去Executor这个接口类
public interface Executor {
//其他方法暂时删除 ，这里只看Transaction 这个方法，然后看他的实现类
Transaction getTransaction();
}
然后跟进去 是一个 抽象类 BaseExecutor，然后一堆的 构造方法以及实现，如下
public abstract class BaseExecutor implements Executor {
private static final Log log = LogFactory.getLog(BaseExecutor.class);
protected Transaction transaction;
protected Executor wrapper;
protected ConcurrentLinkedQueue deferredLoads;
protected PerpetualCache localCache;
protected PerpetualCache localOutputParameterCache;
protected Configuration configuration;
protected int queryStack;
private boolean closed;
protected BaseExecutor(Configuration configuration, Transaction transaction) {
this.transaction = transaction;
this.deferredLoads = new ConcurrentLinkedQueue<>();
this.localCache = new PerpetualCache("LocalCache");
this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
this.closed = false;
this.configuration = configuration;
this.wrapper = this;
}
@Override
public Transaction getTransaction() {
if (closed) {
throw new ExecutorException("Executor was closed.");
}
return transaction;
}
@Override
public void close(boolean forceRollback) {
try {
try {
rollback(forceRollback);
} finally {
if (transaction != null) {
transaction.close();
}
}
} catch (SQLException e) {
// Ignore. There's nothing that can be done at this point.
log.warn("Unexpected exception on closing transaction. Cause: " + e);
} finally {
transaction = null;
deferredLoads = null;
localCache = null;
localOutputParameterCache = null;
closed = true;
}
}
@Override
public boolean isClosed() {
return closed;
}
@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.");
}
clearLocalCache();
return doUpdate(ms, parameter);
}
@Override
public List flushStatements() throws SQLException {
return flushStatements(false);
}
public List flushStatements(boolean isRollBack) throws SQLException {
if (closed) {
throw new ExecutorException("Executor was closed.");
}
return doFlushStatements(isRollBack);
}
@Override
public List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameter);
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
@SuppressWarnings("unchecked")
@Override
public List 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 list;
try {
queryStack++;
list = resultHandler == null ? (List) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
queryStack--;
}
if (queryStack == 0) {
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
return list;
}
@Override
public Cursor queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameter);
return doQueryCursor(ms, parameter, rowBounds, boundSql);
}
@Override
public void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class targetType) {
if (closed) {
throw new ExecutorException("Executor was closed.");
}
DeferredLoad deferredLoad = new DeferredLoad(resultObject, property, key, localCache, configuration, targetType);
if (deferredLoad.canLoad()) {
deferredLoad.load();
} else {
deferredLoads.add(new DeferredLoad(resultObject, property, key, localCache, configuration, targetType));
}
}
@Override
public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
if (closed) {
throw new ExecutorException("Executor was closed.");
}
CacheKey cacheKey = new CacheKey();
cacheKey.update(ms.getId());
cacheKey.update(rowBounds.getOffset());
cacheKey.update(rowBounds.getLimit());
cacheKey.update(boundSql.getSql());
List parameterMappings = boundSql.getParameterMappings();
TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
// mimic DefaultParameterHandler logic
for (ParameterMapping parameterMapping : parameterMappings) {
if (parameterMapping.getMode() != ParameterMode.OUT) {
Object value;
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);
}
cacheKey.update(value);
}
}
if (configuration.getEnvironment() != null) {
// issue #176
cacheKey.update(configuration.getEnvironment().getId());
}
return cacheKey;
}
@Override
public boolean isCached(MappedStatement ms, CacheKey key) {
return localCache.getObject(key) != null;
}
@Override
public void commit(boolean required) throws SQLException {
if (closed) {
throw new ExecutorException("Cannot commit, transaction is already closed");
}
clearLocalCache();
flushStatements();
if (required) {
transaction.commit();
}
}
@Override
public void rollback(boolean required) throws SQLException {
if (!closed) {
try {
clearLocalCache();
flushStatements(true);
} finally {
if (required) {
transaction.rollback();
}
}
}
}
@Override
public void clearLocalCache() {
if (!closed) {
localCache.clear();
localOutputParameterCache.clear();
}
}
protected abstract int doUpdate(MappedStatement ms, Object parameter)
throws SQLException;
protected abstract List doFlushStatements(boolean isRollback)
throws SQLException;
protected abstract List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
throws SQLException;
protected abstract Cursor doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql)
throws SQLException;
protected void closeStatement(Statement statement) {
if (statement != null) {
try {
statement.close();
} catch (SQLException e) {
// ignore
}
}
}
/**
* Apply a transaction timeout.
* @param statement a current statement
* @throws SQLException if a database access error occurs, this method is called on a closed Statement
* @since 3.4.0
* @see StatementUtil#applyTransactionTimeout(Statement, Integer, Integer)
*/
protected void applyTransactionTimeout(Statement statement) throws SQLException {
StatementUtil.applyTransactionTimeout(statement, statement.getQueryTimeout(), transaction.getTimeout());
}
private void handleLocallyCachedOutputParameters(MappedStatement ms, CacheKey key, Object parameter, BoundSql boundSql) {
if (ms.getStatementType() == StatementType.CALLABLE) {
final Object cachedParameter = localOutputParameterCache.getObject(key);
if (cachedParameter != null && parameter != null) {
final MetaObject metaCachedParameter = configuration.newMetaObject(cachedParameter);
final MetaObject metaParameter = configuration.newMetaObject(parameter);
for (ParameterMapping parameterMapping : boundSql.getParameterMappings()) {
if (parameterMapping.getMode() != ParameterMode.IN) {
final String parameterName = parameterMapping.getProperty();
final Object cachedValue = metaCachedParameter.getValue(parameterName);
metaParameter.setValue(parameterName, cachedValue);
}
}
}
}
}
private List queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List list;
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}
protected Connection getConnection(Log statementLog) throws SQLException {
Connection connection = transaction.getConnection();
if (statementLog.isDebugEnabled()) {
return ConnectionLogger.newInstance(connection, statementLog, queryStack);
} else {
return connection;
}
}
@Override
public void setExecutorWrapper(Executor wrapper) {
this.wrapper = wrapper;
}
private static class DeferredLoad {
private final MetaObject resultObject;
private final String property;
private final Class targetType;
private final CacheKey key;
private final PerpetualCache localCache;
private final ObjectFactory objectFactory;
private final ResultExtractor resultExtractor;
// issue #781
public DeferredLoad(MetaObject resultObject,
String property,
CacheKey key,
PerpetualCache localCache,
Configuration configuration,
Class targetType) {
this.resultObject = resultObject;
this.property = property;
this.key = key;
this.localCache = localCache;
this.objectFactory = configuration.getObjectFactory();
this.resultExtractor = new ResultExtractor(configuration, objectFactory);
this.targetType = targetType;
}
public boolean canLoad() {
return localCache.getObject(key) != null && localCache.getObject(key) != EXECUTION_PLACEHOLDER;
}
public void load() {
@SuppressWarnings("unchecked")
// we suppose we get back a List
List
上面 贴了源码，篇幅太长，这里把上面源码中PerpetualCache这个类给单独跳出来，我们看看
这个PerpetualCache类 就是mybaits 的缓存存放类，这里是通过 实现Cache接口，然后通过HashMap进行实现的
，具体代码如下：
public class PerpetualCache implements Cache {
private final String id;
private Map cache = new HashMap<>();
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) {
cache.put(key, value);
}
@Override
public Object getObject(Object key) {
return cache.get(key);
}
@Override
public Object removeObject(Object key) {
return cache.remove(key);
}
@Override
public void clear() {
cache.clear();
}
@Override
public boolean equals(Object o) {
if (getId() == null) {
throw new CacheException("Cache instances require an ID.");
}
if (this == o) {
return true;
}
if (!(o instanceof Cache)) {
return false;
}
Cache otherCache = (Cache) o;
return getId().equals(otherCache.getId());
}
@Override
public int hashCode() {
if (getId() == null) {
throw new CacheException("Cache instances require an ID.");
}
return getId().hashCode();
}
}
到此为止，就揭盖了，mybatis底层的缓存实现原来就是我们熟知 HashMap类，那么一级缓存和二级缓存，到底是怎么样的呢， 那我们继续看源码
刚才上面看到这个BaseExecutor类的queryFromDatabase方法 ，就是这个方法，当我们第一次执行sql 的时候就会把sql缓存到perpetualCache类中，就是这句：localCache.putObject();
privateList queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { List list; localCache.putObject(key, EXECUTION_PLACEHOLDER); try { list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); } finally { localCache.removeObject(key); } localCache.putObject(key, list); if (ms.getStatementType() == StatementType.CALLABLE) { localOutputParameterCache.putObject(key, parameter); } return list; }
当我们是同一条sql，第二次查询的，就会直接从缓存里面获取，而不会再次查询数据库，查询缓存的时候，也是先从二级缓存中查询，要是二级缓存没有，在从一级缓存中获取。
一般正式生产环境会配置三级缓存（（比如redist，没有配置或者其他第三方））
只有在 INSERT、UPDATE 、delete 提交之后会清空缓存
mybatis 要想开启可以配置两种方法
（1）配置文件mybatis-config.xml加入
（2）xxxxxmapper.xml文件中
