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MyBatis源码分析之@SelectProvider注解使用详解
之前讲了MyBatis的配置、plugin、Select查询，还有@MapKey注解的使用与原理，还有返回@ResultMap等等，我原想直接从MyBatis的缓存开始说起，但是想想还是得说一下MyBatis中的@selectProvider，这个注解我也是在用了好久的MyBatis才用到，功能就是用来单独写一个class类与方法，用来提供一些xml或者注解中不好写的sql，今天就来说下这个注解的具体用法与源码。
@SelectProvider注解用法
写一个简单的@SelectProvider的用法，新建class类，添加一个根据userId查询user的方法。
SelectSqlProvider:
public class SelectSqlProvider {
public String selectByUserId(Long id) {
StringBuffer buffer = new StringBuffer();
buffer.append("SELECT * FROM user where ");
buffer.append("id = ").append(id).append(";");
return buffer.toString();
}
}
SelectSqlProvider中提供了一个很简单的查询方法，根据userId返回user对象，里面就是用了一个StringBuffer对象来拼接一个SQL语句，我想更多的是想用MyBatis中的SQL Builder的写法，SQL Builder写法在官方网站地址为http://www.mybatis.org/mybatis-3/zh/statement-builders.html，不得不说SQL Builder的写法确实比较漂亮，很工整，不过也是看自己运用的熟练程度吧。
UserMapper:
@ResultMap("BaseResultMap")
@SelectProvider(type = SelectSqlProvider.class, method = "selectByUserId")
User getUserByUserId(long id);
mapper中的其他方法就不贴出来了，需要说的就是这一个，这一个方法在xml中没有对应的sql，在该方法上也没有@Select注解修饰，只有@SelectProvider注解，@SelectProvider中两个属性，type为提供sql的class类，method为指定方法。
对应Mapper的调用与结果在这就不再分析了，就是简单的返回user对象，下文将是对@SelectProvider注解作用的详解。
2. @SelectProvider源码分析
说起Select查询，基本就又是回到我们先前那几篇文章说的了，@SelectProvider注解加载问题，之前的文章中说了如何在解析xml之后解析注解中的SQL，这一种无非换了种样式，从由注解提供改为了从class类中单独写方法提供SQL，我们来看下相关源码实现。
这里就还要回到mapper的解析处，回到开始的parseConfiguration方法中mapperElement。
mapperElement(root.evalNode("mappers"));
这一行在解析xml文件之后，最后进行了addMapper操作。
private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
if ("package".equals(child.getName())) {
String mapperPackage = child.getStringAttribute("name");
configuration.addMappers(mapperPackage);
} else {
String resource = child.getStringAttribute("resource");
String url = child.getStringAttribute("url");
String mapperClass = child.getStringAttribute("class");
if (resource != null && url == null && mapperClass == null) {
ErrorContext.instance().resource(resource);
InputStream inputStream = Resources.getResourceAsStream(resource);
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
mapperParser.parse();
} else if (resource == null && url != null && mapperClass == null) {
ErrorContext.instance().resource(url);
InputStream inputStream = Resources.getUrlAsStream(url);
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
mapperParser.parse();
} else if (resource == null && url == null && mapperClass != null) {
Class<?> mapperInterface = Resources.classForName(mapperClass);
configuration.addMapper(mapperInterface);
} else {
throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
}
}
}
}
}
但是从前文中我们知addMapper操作不仅将mapper保存进knownMappers中，并且还进行了注解Mapper的解析，从而实现了对注解sql的加载，同时**@SelectProvider**也是在这里进行加载的。
knownMappers.put(type, new MapperProxyFactory<T>(type));
// It's important that the type is added before the parser is run
// otherwise the binding may automatically be attempted by the
// mapper parser. If the type is already known, it won't try.
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
进入到parse方法中,parse方法最终转到parseStatement方法,在parseStatement方法中，在获取SqlSource对象时，对method方法进行了进一步的解析。
SqlSource sqlSource = getSqlSourceFromAnnotations(method, parameterTypeClass, languageDriver);
private SqlSource getSqlSourceFromAnnotations(Method method, Class<?> parameterType, LanguageDriver languageDriver) {
try {
Class<? extends Annotation> sqlAnnotationType = getSqlAnnotationType(method);
Class<? extends Annotation> sqlProviderAnnotationType = getSqlProviderAnnotationType(method);
if (sqlAnnotationType != null) {
if (sqlProviderAnnotationType != null) {
throw new BindingException("You cannot supply both a static SQL and SqlProvider to method named " + method.getName());
}
Annotation sqlAnnotation = method.getAnnotation(sqlAnnotationType);
final String[] strings = (String[]) sqlAnnotation.getClass().getMethod("value").invoke(sqlAnnotation);
return buildSqlSourceFromStrings(strings, parameterType, languageDriver);
} else if (sqlProviderAnnotationType != null) {
Annotation sqlProviderAnnotation = method.getAnnotation(sqlProviderAnnotationType);
return new ProviderSqlSource(assistant.getConfiguration(), sqlProviderAnnotation, type, method);
}
return null;
} catch (Exception e) {
throw new BuilderException("Could not find value method on SQL annotation. Cause: " + e, e);
}
}
这里可以加上断点，对我们上面写的代码调试一下,如下图。
到这一步就是对@SelectProvider注解的解析，可以看到此时的method方法为getUserByUserId。type类型为UserMapper等等。我们继续进入到ProviderSqlSource中，看看是如何组装sql的。
public ProviderSqlSource(Configuration configuration, Object provider, Class<?> mapperType, Method mapperMethod) {
String providerMethodName;
try {
this.configuration = configuration;
this.sqlSourceParser = new SqlSourceBuilder(configuration);
this.providerType = (Class<?>) provider.getClass().getMethod("type").invoke(provider);
providerMethodName = (String) provider.getClass().getMethod("method").invoke(provider);
for (Method m : this.providerType.getMethods()) {
if (providerMethodName.equals(m.getName()) && CharSequence.class.isAssignableFrom(m.getReturnType())) {
if (providerMethod != null){
throw new BuilderException("Error creating SqlSource for SqlProvider. Method '"
+ providerMethodName + "' is found multiple in SqlProvider '" + this.providerType.getName()
+ "'. Sql provider method can not overload.");
}
this.providerMethod = m;
this.providerMethodArgumentNames = new ParamNameResolver(configuration, m).getNames();
this.providerMethodParameterTypes = m.getParameterTypes();
}
}
} catch (BuilderException e) {
throw e;
} catch (Exception e) {
throw new BuilderException("Error creating SqlSource for SqlProvider. Cause: " + e, e);
}
if (this.providerMethod == null) {
throw new BuilderException("Error creating SqlSource for SqlProvider. Method '"
+ providerMethodName + "' not found in SqlProvider '" + this.providerType.getName() + "'.");
}
for (int i = 0; i< this.providerMethodParameterTypes.length; i++) {
Class<?> parameterType = this.providerMethodParameterTypes[i];
if (parameterType == ProviderContext.class) {
if (this.providerContext != null){
throw new BuilderException("Error creating SqlSource for SqlProvider. ProviderContext found multiple in SqlProvider method ("
+ this.providerType.getName() + "." + providerMethod.getName()
+ "). ProviderContext can not define multiple in SqlProvider method argument.");
}
this.providerContext = new ProviderContext(mapperType, mapperMethod);
this.providerContextIndex = i;
}
}
}
此处对sqlSourceParser与providerType、providerMethodName等参数进行了实例化与赋值，最后返回sqlSource对象。
此处得到的可以说还不是原有的sql，所以在Select查询的时候，还要继续追踪看一下到底是如何执行sql的，这就要继续回到Select查询方法了，在前面很多文章中知最后查询调用基本都是调用的selectList方法，此处还是要从这里分析开始。
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
进入到executor.query方法中,executor的实现有两种，一种是BaseExecutor，一种是CacheingExecutor，而这种的初始化条件为openSession中的newExecutor方法。
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) {
executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
这里一般就是实例化为Simple类型，但是如果cacheEnable字段为true的话，返回CachingExecutor对象。而cacheEnable字段算得上是之前漏说了的一个属性，这个是在loadSettings时进行初始化的，而如果没有设置cacheEnable字段时，默认设置为true，如下：
private void settingsElement(Properties props) throws Exception { configuration.setAutoMappingUnknownColumnBehavior(AutoMappingUnknownColumnBehavior.valueOf(props.getProperty("autoMappingUnknownColumnBehavior", "NONE")));
configuration.setCacheEnabled(booleanValueOf(props.getProperty("cacheEnabled"), true));
configuration.setProxyFactory((ProxyFactory) createInstance(props.getProperty("proxyFactory")))
}
在说完BaseExecutor和CacheingExecutor之后，此处继续回到query方法。
@Override
public <E> List<E> 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);
}
在query方法中获取到boundSql对象，此处可以调试一下代码，看看boundSql中有什么参数。
此处已经完成了sql的组装，继续getBoundSql看看进行了什么操作。
public BoundSql getBoundSql(Object parameterObject) {
BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings == null || parameterMappings.isEmpty()) {
boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject);
}
// check for nested result maps in parameter mappings (issue #30)
for (ParameterMapping pm : boundSql.getParameterMappings()) {
String rmId = pm.getResultMapId();
if (rmId != null) {
ResultMap rm = configuration.getResultMap(rmId);
if (rm != null) {
hasNestedResultMaps |= rm.hasNestedResultMaps();
}
}
}
return boundSql;
}
还需要继续追溯sqlSource.getBoundSql(parameterObject)，此处SqlSource毫无疑问为ProviderSqlSource类。
@Override
public BoundSql getBoundSql(Object parameterObject) {
SqlSource sqlSource = createSqlSource(parameterObject);
return sqlSource.getBoundSql(parameterObject);
}
private SqlSource createSqlSource(Object parameterObject) {
try {
int bindParameterCount = providerMethodParameterTypes.length - (providerContext == null ? 0 : 1);
String sql;
if (providerMethodParameterTypes.length == 0) {
sql = invokeProviderMethod();
} else if (bindParameterCount == 0) {
sql = invokeProviderMethod(providerContext);
} else if (bindParameterCount == 1 &&
(parameterObject == null || providerMethodParameterTypes[(providerContextIndex == null || providerContextIndex == 1) ? 0 : 1].isAssignableFrom(parameterObject.getClass()))) {
sql = invokeProviderMethod(extractProviderMethodArguments(parameterObject));
} else if (parameterObject instanceof Map) {
@SuppressWarnings("unchecked")
Map<String, Object> params = (Map<String, Object>) parameterObject;
sql = invokeProviderMethod(extractProviderMethodArguments(params, providerMethodArgumentNames));
} else {
throw new BuilderException("Error invoking SqlProvider method ("
+ providerType.getName() + "." + providerMethod.getName()
+ "). Cannot invoke a method that holds "
+ (bindParameterCount == 1 ? "named argument(@Param)": "multiple arguments")
+ " using a specifying parameterObject. In this case, please specify a 'java.util.Map' object.");
}
Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
return sqlSourceParser.parse(replacePlaceholder(sql), parameterType, new HashMap<String, Object>());
} catch (BuilderException e) {
throw e;
} catch (Exception e) {
throw new BuilderException("Error invoking SqlProvider method ("
+ providerType.getName() + "." + providerMethod.getName()
+ "). Cause: " + e, e);
}
}
createSqlSource方法有些意思，对参数个数进行了校验，如果是没有参数，直接执行invokeProviderMethod()方法，如果是一个则进行传参，如果多个判断当前类型是否是Map类型，否则抛错，等会倒是可以测试一下，这里看下invokeProviderMethod方法。
private String invokeProviderMethod(Object... args) throws Exception {
Object targetObject = null;
if (!Modifier.isStatic(providerMethod.getModifiers())) {
targetObject = providerType.newInstance();
}
CharSequence sql = (CharSequence) providerMethod.invoke(targetObject, args);
return sql != null ? sql.toString() : null;
}
invokeProviderMethod方法其实就没多少可说的了,对当前方法、对象进行了一个反射获取值的操作，从而拿到对应sql。
在获取到sql之后剩下的执行就和常规的是一样的了，这里就不再继续说后面的东西了。