- mabatis 如何解析mapper.xml文件里的if等标签，生成执行语句？设置参数？
- 下面通过跟踪源码我们来查看下具体是如何执行。
- 这里我们可以看到我们的mapper接口是通过MapperProxy 生成的代理对象进行调用的。即诸如的是一个动态代理对象。
public class MapperProxy<T> implements InvocationHandler, Serializable {
// 反射执行方法
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (method.isDefault()) {
if (privateLookupInMethod == null) {
return invokeDefaultMethodJava8(proxy, method, args);
} else {
return invokeDefaultMethodJava9(proxy, method, args);
}
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
// 生成并缓存MapperMethod 对象。method 为key,value 是MapperMethod 。这样就建立了method到
//MapperMethod 的映射关系。
final MapperMethod mapperMethod = cachedMapperMethod(method);
// 执行调用
return mapperMethod.execute(sqlSession, args);
}
}
public class MapperMethod {
//
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
// 根据不同命令执行不同的方法
switch (command.getType()) {
case INSERT: {
// 传入的参数转换
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
if (method.returnsOptional()
&& (result == null || !method.getReturnType().equals(result.getClass()))) {
result = Optional.ofNullable(result);
}
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}
}
private class SqlSessionInterceptor implements InvocationHandler {
// 拦截并获取sqlsession执行调用
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
SqlSession sqlSession = getSqlSession(SqlSessionTemplate.this.sqlSessionFactory,
SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);
try {
Object result = method.invoke(sqlSession, args);
if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
// force commit even on non-dirty sessions because some databases require
// a commit/rollback before calling close()
sqlSession.commit(true);
}
return result;
} catch (Throwable t) {
Throwable unwrapped = unwrapThrowable(t);
if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
// release the connection to avoid a deadlock if the translator is no loaded. See issue #22
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
sqlSession = null;
Throwable translated = SqlSessionTemplate.this.exceptionTranslator
.translateExceptionIfPossible((PersistenceException) unwrapped);
if (translated != null) {
unwrapped = translated;
}
}
throw unwrapped;
} finally {
if (sqlSession != null) {
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
}
}
}
}
public class DefaultSqlSession implements SqlSession {
@Override
public int update(String statement, Object parameter) {
try {
dirty = true;
MappedStatement ms = configuration.getMappedStatement(statement);
// 执行器调用update方法做更新
return executor.update(ms, wrapCollection(parameter));
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error updating database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
public class CachingExecutor implements Executor {
// 缓存执行器调用的是其他是那种执行器的一种，即简单执行器 ，批量执行器，reuse 执行器。
private final Executor delegate;
//缓存执行器，默认二级缓存打开。
@Override
public int update(MappedStatement ms, Object parameterObject) throws SQLException {
flushCacheIfRequired(ms);
return delegate.update(ms, parameterObject);
}
}
// 向下继续追踪，我们可以看到sqlNode 节点是我们把标签解析为sql的地方。静态sql。
public class StaticTextSqlNode implements SqlNode {
private final String text;
public StaticTextSqlNode(String text) {
this.text = text;
}
@Override
public boolean apply(DynamicContext context) {
//静态sql直接添加
context.appendSql(text);
return true;
}
}
}
// foreach 标签解析
public class ForEachSqlNode implements SqlNode {
public static final String ITEM_PREFIX = "__frch_";
public boolean apply(DynamicContext context) {
Map<String, Object> bindings = context.getBindings();
final Iterable<?> iterable = evaluator.evaluateIterable(collectionExpression, bindings);
if (!iterable.iterator().hasNext()) {
return true;
}
boolean first = true;
applyOpen(context);
int i = 0;
for (Object o : iterable) {
DynamicContext oldContext = context;
if (first || separator == null) {
context = new PrefixedContext(context, "");
} else {
context = new PrefixedContext(context, separator);
}
int uniqueNumber = context.getUniqueNumber();
// Issue #709
if (o instanceof Map.Entry) {
@SuppressWarnings("unchecked")
Map.Entry<Object, Object> mapEntry = (Map.Entry<Object, Object>) o;
applyIndex(context, mapEntry.getKey(), uniqueNumber);
applyItem(context, mapEntry.getValue(), uniqueNumber);
} else {
applyIndex(context, i, uniqueNumber);
applyItem(context, o, uniqueNumber);
}
contents.apply(new FilteredDynamicContext(configuration, context, index, item, uniqueNumber));
if (first) {
first = !((PrefixedContext) context).isPrefixApplied();
}
context = oldContext;
i++;
}
applyClose(context);
context.getBindings().remove(item);
context.getBindings().remove(index);
return true;
}
}
// 标签解析的根
public class MixedSqlNode implements SqlNode {
private final List<SqlNode> contents;
public MixedSqlNode(List<SqlNode> contents) {
this.contents = contents;
}
@Override
public boolean apply(DynamicContext context) {
// 循环解析
contents.forEach(node -> node.apply(context));
return true;
}
}
// DynamicSqlSource 的getBoundSql 方法
public BoundSql getBoundSql(Object parameterObject) {
DynamicContext context = new DynamicContext(configuration, parameterObject);
rootSqlNode.apply(context);
SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
// parse解析，将形参去除，形成带？的sql.
SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());
//构建携带参数，带？ sql 的对象。
BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
context.getBindings().forEach(boundSql::setAdditionalParameter);
return boundSql;
}
// DefaultParameterHandler 的参数设置方法
public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings != null) {
for (int i = 0; i < parameterMappings.size(); i++) {
ParameterMapping parameterMapping = parameterMappings.get(i);
if (parameterMapping.getMode() != ParameterMode.OUT) {
Object value;
String propertyName = parameterMapping.getProperty();
if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
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);
}
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if (value == null && jdbcType == null) {
jdbcType = configuration.getJdbcTypeForNull();
}
try {
typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException | SQLException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
}
}
}
}
}
// 类型转换器，参数设置方法 BaseTypeHandler jdbc 类型到java类型的转换
@Override
public void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException {
if (parameter == null) {
if (jdbcType == null) {
throw new TypeException("JDBC requires that the JdbcType must be specified for all nullable parameters.");
}
try {
ps.setNull(i, jdbcType.TYPE_CODE);
} catch (SQLException e) {
throw new TypeException("Error setting null for parameter #" + i + " with JdbcType " + jdbcType + " . "
+ "Try setting a different JdbcType for this parameter or a different jdbcTypeForNull configuration property. "
+ "Cause: " + e, e);
}
} else {
try {
setNonNullParameter(ps, i, parameter, jdbcType);
} catch (Exception e) {
throw new TypeException("Error setting non null for parameter #" + i + " with JdbcType " + jdbcType + " . "
+ "Try setting a different JdbcType for this parameter or a different configuration property. "
+ "Cause: " + e, e);
}
}
}
本文探讨Mybatis如何解析mapper.xml文件中的if等标签并生成执行语句，以及参数设置的过程。通过源码分析，我们发现MapperProxy生成的代理对象执行接口调用，DynamicSqlSource持有解析后的SqlNode，标签解析后的结果存储在rootSqlNode中。类型转换器如BaseTypeHandler处理参数设置。Mybatis的核心包括Configuration、MapperProxy、MapperRegistry、MapperProxyFactory和MapperMethod等类，执行器如SimpleExecutor、ReuseExecutor、BatchExecutor和CachingExecutor各自有不同的功能。二级缓存由TransactionalCacheManage管理。理解这些概念有助于解答面试问题，例如Mybatis的执行器种类及其差异，以及#和$的区别。
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