首先，我们看使用原生的JDBC来操作数据库的方式：
// 1. 获取JDBC Connection
Connection connection = DbManager.getConnectoin();
// 2. 组装sql语句
String sql = "insert into temp(id, name) values(?,?)";
// 3. 构造 PreparedStatement
PreparedStatement ps = connection.prepareStatement(sql);
// 4. 为 PreparedStatement 设置参数
ps.setLong(1, 1L);
ps.setString(2, "aaa");
// 5. 执行 PreparedStatement
int count = ps.executeUpdate();
那么，MyBatis是如何对上面的过程进行封装的呢？
我们以update为例，看MyBatis是如何封装这几个步骤的：
sql执行的入口：
DefaultSqlSession.update(String statement, Object parameter) --> Executor.update(MappedStatement ms, Object parameter) -->
SimpleExecutor.doUpdate(MappedStatement ms, Object parameter)
DefaultSqlSession.update(String, Object)
public int update(String statement, Object parameter) {
try {
dirty = true;
MappedStatement ms = configuration.getMappedStatement(statement); // 将mapper中的sql解析成原生的JDBC能够识别的sql，其中包括MyBatis标签的解析，"#{key,JdbcType=XXX}"占位符替换成"?"
return executor.update(ms, wrapCollection(parameter)); // 构造java.sql.PreparedStatement --> 往java.sql.PreparedStatement设置sql的参数 --> 执行sql语句
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error updating database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
可以看出，MyBatis将这个过程封装成了两步：
1. sql语句解析 （解析sql，替换占位符为"?"）
2. sql语句执行 （设置sql中的参数，执行sql）
### 1. sql语句解析
configuration.getMappedStatement(statement)会去解析MyBatis的标签（比如：<foreach>、<if>等），并且将"#{key, JdbcType=XXX}"替换成原生的JDBC看的懂的占位符"?"。
流程如下：
Configuration.getMappedStatement(String, boolean) --> Configuration.buildAllStatements() --> MapperAnnotationBuilder.parseStatement(Method) -->
MapperAnnotationBuilder.buildSqlSourceFromStrings(String[], Class<?>, LanguageDriver) --> RawLanguageDriver.createSqlSource(Configuration, String, Class<?>)
--> XMLLanguageDriver.createSqlSource(Configuration, String, Class<?>)
20170721更新：（xml应该是如下）
Configuration.getMappedStatement(String, boolean) --> Configuration.buildAllStatements() --> XMLStatementBuilder.parseStatementNode() -->
LanguageDriver.createSqlSource(Configuration configuration, XNode script, Class<?> parameterType) --> RawLanguageDriver.createSqlSource(Configuration, String, Class<?>)
--> XMLLanguageDriver.createSqlSource(Configuration, String, Class<?>)
可以看出，MyBatis中的动态SQL解析是通过别名为 xml 语言驱动器 org.apache.ibatis.scripting.xmltags.XmlLanguageDriver 驱动解析的。
public class XMLLanguageDriver implements LanguageDriver {
public ParameterHandler createParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
return new DefaultParameterHandler(mappedStatement, parameterObject, boundSql);
}
public SqlSource createSqlSource(Configuration configuration, XNode script, Class<?> parameterType) {
XMLScriptBuilder builder = new XMLScriptBuilder(configuration, script, parameterType);
return builder.parseScriptNode();
}
public SqlSource createSqlSource(Configuration configuration, String script, Class<?> parameterType) {
if (script.startsWith("<script>")) { // issue #3
XPathParser parser = new XPathParser(script, false, configuration.getVariables(), new XMLMapperEntityResolver());
return createSqlSource(configuration, parser.evalNode("/script"), parameterType);
} else {
script = PropertyParser.parse(script, configuration.getVariables()); // issue #127
TextSqlNode textSqlNode = new TextSqlNode(script);
if (textSqlNode.isDynamic()) { // 动态sql解析入口
return new DynamicSqlSource(configuration, textSqlNode);
} else {
return new RawSqlSource(configuration, script, parameterType);
}
}
}
}
DynamicSqlSource.java
public class DynamicSqlSource implements SqlSource {
private Configuration configuration;
private SqlNode rootSqlNode;
public DynamicSqlSource(Configuration configuration, SqlNode rootSqlNode) {
this.configuration = configuration;
this.rootSqlNode = rootSqlNode;
}
public BoundSql getBoundSql(Object parameterObject) {
DynamicContext context = new DynamicContext(configuration, parameterObject); // 构造DynamicContext，并将DynamicContext中的属性ContextMap bindings赋值
rootSqlNode.apply(context); // 处理sql中的标签，比如：<foreach>、<if>等
SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings()); // 将占位符"#{key, jdbcType=XXX}"替换成占位符"?"。也就是原生的java.sql.PreparedStatement支持的占位符。（具体是通过GenericTokenParser#parse(String text)来实现的）
BoundSql boundSql = sqlSource.getBoundSql(parameterObject); // 构造BoundSql
for (Map.Entry<String, Object> entry : context.getBindings().entrySet()) {
boundSql.setAdditionalParameter(entry.getKey(), entry.getValue()); // 将参数的key和value存放至BoundSql的MetaObject metaParameters;中
}
return boundSql;
}
}
####2. sql语句执行
通过SimpleExecutor.java 去执行sql语句。其中，最重要的两步是 1. prepareStatement(handler, ms.getStatementLog()); 2. handler.update(stmt);
public class SimpleExecutor extends BaseExecutor {
public SimpleExecutor(Configuration configuration, Transaction transaction) {
super(configuration, transaction);
}
// 所有的数据库更新操作，都会调用doUpdate()，包括insert、update、delete
public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null); // 构造执行Statement的StatementHandler
stmt = prepareStatement(handler, ms.getStatementLog()); // 通过java.sql.Connection拿到java.sql.PreparedStatement，并且为 java.sql.PreparedStatement 设置sql中的参数
return handler.update(stmt); // 代理执行 java.sql.PreparedStatement （PreparedStatementHandler.update(Statement)）
} finally {
closeStatement(stmt);
}
}
// 所有的数据库查询操作，都用调用doQuery()
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
return Collections.emptyList();
}
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection); // 通过 java.sql.Connection 拿到 java.sql.PreparedStatement （PreparedStatementHandler.instantiateStatement(Connection)）
handler.parameterize(stmt); // 为 java.sql.PreparedStatement 设置sql中的参数。（DefaultParameterHandler.setParameters(PreparedStatement)）
return stmt;
}
}
至此，MyBatis的封装就一目了然了。^_^
附：
MyBatis开发文档： http://www.mybatis.org/mybatis-3/zh/dynamic-sql.html