1、mybatis擴展點plugins
mybatis的擴展是通過攔截器Interceptor來實現的,本質刪就是JDK的動態代理,所以它只能對接口進行攔截,mybatis可以對以下四個接口類型進行攔截,也就是說會對這4種對象進行代理,所有的代理攔截都是通過 InterceptorChain.pluginAll(Object target) 來實現的。
Executor: 執行器,執行SQL語句(所有的sql都通過它來執行),並且對事務、緩存等提供統一接口。(在這一層上做攔截的權限會更大)
StatementHandler: 對statement進行預處理,並且提供統一的原子的增、刪、改、查接口。(如果要在SQL執行前進行攔截的話,攔截這裏就可以了)
ResultSetHandler:對返回結果ResultSet進行處理。
PameterHandler:對參數進行賦值。
SqlSession的創建過程:
mybatis中的SQL都是通過DefaultSqlSession去執行的,其創建過程如下:
// SqlSessionFactoryBuilder => DefaultSqlSessionFactory => DefaultSqlSession
SqlSession sqlSession = new SqlSessionFactoryBuilder().build(in, "development", pro).openSession();
2、源碼解讀具體實現(以Executor接口爲例)
2.1、創建SqlSession時,SqlSessionFactroy會解析mybatis.xml配置文件中的plugins標籤,並將Interceptor屬性定義的Interceptor放到interceptorChain中;
// SqlSessionFactoryBuilder.java
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
// 解析mybatis.xml配置文件,並創建DefaultSqlSessionFactory
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
reader.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}
// XMLConfigBuilder.java
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
// 解析mybatis.xml中的各個標籤
private void parseConfiguration(XNode root) {
try {
propertiesElement(root.evalNode("properties")); //issue #117 read properties first
typeAliasesElement(root.evalNode("typeAliases"));
pluginElement(root.evalNode("plugins"));
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
settingsElement(root.evalNode("settings"));
environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
typeHandlerElement(root.evalNode("typeHandlers"));
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
// 解析plugins標籤,並把Interceptor放到interceptorChain中
private void pluginElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
String interceptor = child.getStringAttribute("interceptor");
Properties properties = child.getChildrenAsProperties();
Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
interceptorInstance.setProperties(properties);
configuration.addInterceptor(interceptorInstance);
}
}
}
// Configuration,mybatis文件的抽象類
public void addInterceptor(Interceptor interceptor) {
interceptorChain.addInterceptor(interceptor);
}
2.2、DefaultSqlSessionFactory.openSession()時使用JDK動態代理生成@Signature註解指定的被代理類(包含代理的方法以及方法參數)
// DefaultSqlSessionFactory.java
public SqlSession openSession() {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
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);
// 使用Configuration創建Executor
final Executor executor = configuration.newExecutor(tx, execType, autoCommit);
return new DefaultSqlSession(configuration, executor);
} 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();
}
}
// Configuration.java
public Executor newExecutor(Transaction transaction, ExecutorType executorType, boolean autoCommit) {
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) {
executor = new CachingExecutor(executor, autoCommit);
}
// 使用JDK動態代理生成Executor的interceptorChain
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
2.3、InterceptorChain生成的具體過程
// InterceptorChain.java
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}
// Interceptor的具體實現類(即我們業務上要實現的功能)
@Override
public Object plugin(Object arg0) {
return Plugin.wrap(arg0, this);
}
// Plugin.java
public static Object wrap(Object target, Interceptor interceptor) {
// getSignatureMap獲取Interceptor類上的@Intercepts(@Signature)內容
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
// 生成目標類target(Executor.class)的代理類,實現我們需要的plugin功能
if (interfaces.length > 0) {
return Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap));
}
return target;
}
// 解析實現Interceptor接口的類上定義的@Intercepts(@Signature)內容,獲取需要攔截的類和方法。
// 例如:@Signature(type = Executor.class, method = "query", args = { MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})
private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
if (interceptsAnnotation == null) { // issue #251
throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
}
Signature[] sigs = interceptsAnnotation.value();
Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
for (Signature sig : sigs) {
Set<Method> methods = signatureMap.get(sig.type());
if (methods == null) {
methods = new HashSet<Method>();
signatureMap.put(sig.type(), methods);
}
try {
// sig.type()即Executor.class
Method method = sig.type().getMethod(sig.method(), sig.args());
methods.add(method);
} catch (NoSuchMethodException e) {
throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
}
}
return signatureMap;
}
3、mybatis針對各種異常的處理
mybatis通過DefaultSqlSession執行時,會將發生的所有異常統一包裝成PersistenceException再拋出,我們可以通過PersistenceException.getCause()獲取具體的異常。
// DefaultSqlSession.java
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
return result;
} catch (Exception e) {
// 對執行發生的所有Exception進行wrap之後再拋出
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
// ExceptionFactory.java
public static RuntimeException wrapException(String message, Exception e) {
// 將Exception進行統一包裝成PersistenceException
return new PersistenceException(ErrorContext.instance().message(message).cause(e).toString(), e);
}
Ref:
https://www.cnblogs.com/kevin-yuan/p/7219003.html
https://www.cnblogs.com/fangjian0423/p/mybatis-interceptor.html