一、MyBatis拦截器原理探究
1.1 MyBatis拦截器介绍
MyBatis提供了一种插件(plugin)的功能，虽然叫做插件，但其实这是拦截器功能。那么拦截器拦截MyBatis中的哪些内容呢？
MyBatis 允许你在已映射语句执行过程中的某一点进行拦截调用。
默认情况下，MyBatis 允许使用插件来拦截的方法调用包括：
- Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)
- ParameterHandler (getParameterObject, setParameters)
- ResultSetHandler (handleResultSets, handleOutputParameters)
- StatementHandler (prepare, parameterize, batch, update, query)
我们看到了可以拦截Executor接口的部分方法，比如update，query，commit，rollback等方法，还有其他接口的一些方法等。
总体概括为：
- 拦截执行器的方法
- 拦截参数的处理
- 拦截结果集的处理
- 拦截Sql语法构建的处理
1.2 拦截器的使用
首先我们看下MyBatis拦截器的接口定义：
public interface Interceptor {
Object intercept(Invocation invocation) throws Throwable;
Object plugin(Object target);
void setProperties(Properties properties);
}比较简单，只有3个方法。 MyBatis默认没有一个拦截器接口的实现类，开发者们可以实现符合自己需求的拦截器。
下面的MyBatis官网的一个拦截器实例：
@Intercepts({@Signature(
type= Executor.class,
method = "update",
args = {MappedStatement.class,Object.class})})
public class ExamplePlugin implements Interceptor {
public Object intercept(Invocation invocation) throws Throwable {
return invocation.proceed();
}
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}
public void setProperties(Properties properties) {
}
}全局xml配置：
<plugins>
<plugin interceptor="org.format.mybatis.cache.interceptor.ExamplePlugin"></plugin>
</plugins>这个拦截器拦截Executor接口的update方法（其实也就是SqlSession的新增，删除，修改操作），所有执行executor的update方法都会被该拦截器拦截到。
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二、Plugin原理
2.1 Plugin原理
Plugin的实现采用了Java的动态代理，应用了责任链设计模式
2.1.1 InterceptorChain
拦截器链，用于保存从配置文件解析后的所有拦截器
2.1.2 插件链的创建
在Configuration解析配置文件的时候，XMLConfigBuilder.parseConfiguration中会调用pluginElement解析插件信息并实例化后，保存到插件链中
// /configuration/plugins节点
private void pluginElement(XNode parent) throws Exception {
if (parent != null) {
// 获取所有的插件定义
for (XNode child : parent.getChildren()) {
String interceptor = child.getStringAttribute("interceptor");
Properties properties = child.getChildrenAsProperties();
// 反射，实例化插件
Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
interceptorInstance.setProperties(properties);
// 保存到插件链中
configuration.addInterceptor(interceptorInstance);
}
}
}
// Configuration.addInterceptor
public void addInterceptor(Interceptor interceptor) {
interceptorChain.addInterceptor(interceptor);
}
public class InterceptorChain {
// 所有拦截器实例
private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}
public void addInterceptor(Interceptor interceptor) {
interceptors.add(interceptor);
}
public List<Interceptor> getInterceptors() {
return Collections.unmodifiableList(interceptors);
}
}2.1.3 插件拦截
在MyBatis中，只能拦截四种接口的实现类：
- Executor
- ParameterHandler
- ResultSetHandler
- StatementHandler
每种类型的拦截方式都是一样的，这里取executor为例：
在创建SqlSession的时候，会需要创建Executor实现类，在创建时，会调用插件链的加载插件功能：executor = (Executor) interceptorChain.pluginAll(executor);,该方法会形成一个调用链。
// 依次调用每个插件的plugin方法，如果该插件无需拦截target，则直接返回target
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}2.1.3 Plugin
插件代理的实现，这里应用了Java Dynamic Proxy
public class Plugin implements InvocationHandler {
// 需要被代理的实例
private Object target;
// 拦截器实例
private Interceptor interceptor;
// 拦截器需要拦截的方法摘要，这里Class键为Executor等上述的四个
// 值为需要被拦截的方法
private Map<Class<?>, Set<Method>> signatureMap;
// 此类不能直接创建，需要通过静态方法wrap来创建代理类
private Plugin(Object target, Interceptor interceptor,
Map<Class<?>, Set<Method>> signatureMap) {
this.target = target;
this.interceptor = interceptor;
this.signatureMap = signatureMap;
}
public static Object wrap(Object target, Interceptor interceptor) {
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
// 获取需要被代理类的所有待拦截的接口
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
if (interfaces.length > 0) {
// 创建代理类
return Proxy.newProxyInstance(type.getClassLoader(), interfaces,
new Plugin(target, interceptor, signatureMap));
}
// 没有需要拦截的方法，直接返回原实例
return target;
}
// 在代理类中调用
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable {
try {
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
// 判断是否为待拦截方法，这里为动态判断，所有在拦截器多的时候，会影响性能
if (methods != null && methods.contains(method)) {
return interceptor.intercept(new Invocation(target, method,
args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
// 获取需要被拦截的方法摘要
private static Map<Class<?>, Set<Method>> getSignatureMap(
Interceptor interceptor) {
// 先获取拦截器实现类上的注解，提取需要被拦截的方法
/* 注解示例：@Intercepts(value={@Signature(args={Void.class},method="query",type=Void.class)})*/
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 {
// 根据方法名以及参数获取待拦截方法
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;
}
private static Class<?>[] getAllInterfaces(Class<?> type,
Map<Class<?>, Set<Method>> signatureMap) {
Set<Class<?>> interfaces = new HashSet<Class<?>>();
while (type != null) {
for (Class<?> c : type.getInterfaces()) {
if (signatureMap.containsKey(c)) {
interfaces.add(c);
}
}
type = type.getSuperclass();
}
return interfaces.toArray(new Class<?>[interfaces.size()]);
}
}2.1.4 插件配置
在mybatis.xml配置文件：
<plugins>
<plugin interceptor="com.shareinfo.framework.pagination.mybatis.PageInterceptor" />
</plugins>2.1.5 插件实现
// Ibatis 分页拦截器
@Intercepts({ @Signature(type = Executor.class, method = "query", args = { MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class }) })
public class PageInterceptor implements Interceptor
{
static int MAPPED_STATEMENT_INDEX = 0;
static int PARAMETER_INDEX = 1;
static int ROWBOUNDS_INDEX = 2;
static int RESULT_HANDLER_INDEX = 3;
public Object intercept(Invocation invocation) throws Throwable
{
processIntercept(invocation.getArgs());
return invocation.proceed();
}
public void processIntercept(Object[] queryArgs) throws ConfigurationException
{
// 当前环境 MappedStatement，BoundSql，及sql取得
MappedStatement mappedStatement = (MappedStatement)queryArgs[MAPPED_STATEMENT_INDEX];
// 请求的对象
Object parameter = queryArgs[PARAMETER_INDEX];
// 分页信息
RowBounds rowBounds = (RowBounds)queryArgs[ROWBOUNDS_INDEX];
int offset = rowBounds.getOffset();
int limit = rowBounds.getLimit();
if (offset != 0 || limit != Integer.MAX_VALUE)
{
Dialect dialect = getDialect(mappedStatement.getConfiguration());
BoundSql boundSql = mappedStatement.getBoundSql(parameter);
String sql = boundSql.getSql().trim();
sql = dialect.getPaginationSql(sql, offset, limit);
offset = 0; // 这里没有增加的话导致后面分页查询不出来
limit = Integer.MAX_VALUE;
queryArgs[ROWBOUNDS_INDEX] = new RowBounds(offset, limit);
BoundSql newBoundSql = new BoundSql(mappedStatement.getConfiguration(), sql, boundSql.getParameterMappings(), boundSql.getParameterObject());
MappedStatement newMs = copyFromMappedStatement(mappedStatement, new BoundSqlSqlSource(newBoundSql));
queryArgs[MAPPED_STATEMENT_INDEX] = newMs;
}
}
private Dialect getDialect(Configuration configuration) throws ConfigurationException
{
Dialect.Type databaseType = null;
try
{
databaseType = Dialect.Type.valueOf(configuration.getVariables().getProperty("dialect").toUpperCase());
}
catch (Exception e)
{
throw new ConfigurationException("the value of the dialect property in mybatis-config.xml is not defined : " + configuration.getVariables().getProperty("dialect"));
}
Dialect dialect = null;
switch (databaseType)
{
case MYSQL:
dialect = new MySQL5Dialect();
case SQLSERVER:
dialect = new SqlServerDialect();
case ORACLE:
dialect = new OracleDialect();
}
return dialect;
}
private MappedStatement copyFromMappedStatement(MappedStatement ms, SqlSource newSqlSource)
{
Builder builder = new Builder(ms.getConfiguration(), ms.getId(), newSqlSource, ms.getSqlCommandType());
builder.resource(ms.getResource());
builder.fetchSize(ms.getFetchSize());
builder.statementType(ms.getStatementType());
builder.keyGenerator(ms.getKeyGenerator());
// builder.keyProperty(ms.getKeyProperty());
builder.timeout(ms.getTimeout());
builder.parameterMap(ms.getParameterMap());
builder.resultMaps(ms.getResultMaps());
builder.resultSetType(ms.getResultSetType());
builder.cache(ms.getCache());
builder.flushCacheRequired(ms.isFlushCacheRequired());
builder.useCache(ms.isUseCache());
return builder.build();
}
public class BoundSqlSqlSource implements SqlSource
{
BoundSql boundSql;
public BoundSqlSqlSource(BoundSql boundSql)
{
this.boundSql = boundSql;
}
public BoundSql getBoundSql(Object parameterObject)
{
return boundSql;
}
}
public Object plugin(Object arg0)
{
return Plugin.wrap(arg0, this);
}
public void setProperties(Properties properties)
{
}
}