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StatementHandler接口
StatementHandler封装了Mybatis连接数据库操作最基础的部分。因为，无论怎么封装，最终我们都是要使用JDBC和数据库打交道的。
最早我们学习java连接数据库时的代码就像下面写的那样：：
import java.sql.*;
public class FirstExample {
// JDBC driver name and database URL
static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
static final String DB_URL = "jdbc:mysql://localhost/emp";
// Database credentials
static final String USER = "root";
static final String PASS = "123456";
public static void main(String[] args) {
Connection conn = null;
Statement stmt = null;
try{
//STEP 2: Register JDBC driver
Class.forName("com.mysql.jdbc.Driver");
//STEP 3: Open a connection
System.out.println("Connecting to database...");
conn = DriverManager.getConnection(DB_URL,USER,PASS);
//STEP 4: Execute a query
System.out.println("Creating statement...");
stmt = conn.createStatement();
String sql;
sql = "SELECT id, first, last, age FROM Employees";
ResultSet rs = stmt.executeQuery(sql);
//STEP 5: Extract data from result set
while(rs.next()){
//Retrieve by column name
int id = rs.getInt("id");
int age = rs.getInt("age");
String first = rs.getString("first");
String last = rs.getString("last");
//Display values
System.out.print("ID: " + id);
System.out.print(", Age: " + age);
System.out.print(", First: " + first);
System.out.println(", Last: " + last);
}
//STEP 6: Clean-up environment
rs.close();
stmt.close();
conn.close();
}catch(SQLException se){
//Handle errors for JDBC
se.printStackTrace();
}catch(Exception e){
//Handle errors for Class.forName
e.printStackTrace();
}finally{
//finally block used to close resources
try{
if(stmt!=null)
stmt.close();
}catch(SQLException se2){
}// nothing we can do
try{
if(conn!=null)
conn.close();
}catch(SQLException se){
se.printStackTrace();
}//end finally try
}//end try
System.out.println("There are so thing wrong!");
}//end main
}//end FirstExample而对于StatementHandler来说就是将下面的代码进行了封装和抽象，将和数据库交互的能力提供给Mybatis上层应用
StatementHandler接口：
public interface StatementHandler {
// 从connection中获取statement
Statement prepare(Connection connection, Integer transactionTimeout)
throws SQLException;
// 对sql进行设置参数
void parameterize(Statement statement)
throws SQLException;
// 批量执行
void batch(Statement statement)
throws SQLException;
// 执行预编译后的sql语句（update,delete,insert）
int update(Statement statement)
throws SQLException;
// 执行查询sql
List query(Statement statement, ResultHandler resultHandler)
throws SQLException;
// 使用游标执行查询sql
Cursor queryCursor(Statement statement)
throws SQLException;
// 获取执行SQL语句的封装类BoundSql
BoundSql getBoundSql();
// 参数处理器
ParameterHandler getParameterHandler();
} 先看一下接口下面的实现类关系：
BaseStatementHandler
BaseStatementHandler作为继承StatementHandler接口的抽象类存在。
public abstract class BaseStatementHandler implements StatementHandler {
protected final Configuration configuration;
protected final ObjectFactory objectFactory;
protected final TypeHandlerRegistry typeHandlerRegistry;
protected final ResultSetHandler resultSetHandler;
protected final ParameterHandler parameterHandler;
protected final Executor executor;
protected final MappedStatement mappedStatement;
protected final RowBounds rowBounds;
protected BoundSql boundSql;
protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
this.configuration = mappedStatement.getConfiguration();
this.executor = executor;
this.mappedStatement = mappedStatement;
this.rowBounds = rowBounds;
this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
this.objectFactory = configuration.getObjectFactory();
if (boundSql == null) { // issue #435, get the key before calculating the statement
generateKeys(parameterObject);
boundSql = mappedStatement.getBoundSql(parameterObject);
}
this.boundSql = boundSql;
this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
}
@Override
public BoundSql getBoundSql() {
return boundSql;
}
@Override
public ParameterHandler getParameterHandler() {
return parameterHandler;
}
@Override
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
ErrorContext.instance().sql(boundSql.getSql());
Statement statement = null;
try {
statement = instantiateStatement(connection);
setStatementTimeout(statement, transactionTimeout);
setFetchSize(statement);
return statement;
} catch (SQLException e) {
closeStatement(statement);
throw e;
} catch (Exception e) {
closeStatement(statement);
throw new ExecutorException("Error preparing statement. Cause: " + e, e);
}
}
protected abstract Statement instantiateStatement(Connection connection) throws SQLException;
protected void setStatementTimeout(Statement stmt, Integer transactionTimeout) throws SQLException {
Integer queryTimeout = null;
if (mappedStatement.getTimeout() != null) {
queryTimeout = mappedStatement.getTimeout();
} else if (configuration.getDefaultStatementTimeout() != null) {
queryTimeout = configuration.getDefaultStatementTimeout();
}
if (queryTimeout != null) {
stmt.setQueryTimeout(queryTimeout);
}
StatementUtil.applyTransactionTimeout(stmt, queryTimeout, transactionTimeout);
}
protected void setFetchSize(Statement stmt) throws SQLException {
Integer fetchSize = mappedStatement.getFetchSize();
if (fetchSize != null) {
stmt.setFetchSize(fetchSize);
return;
}
Integer defaultFetchSize = configuration.getDefaultFetchSize();
if (defaultFetchSize != null) {
stmt.setFetchSize(defaultFetchSize);
}
}
protected void closeStatement(Statement statement) {
try {
if (statement != null) {
statement.close();
}
} catch (SQLException e) {
//ignore
}
}
protected void generateKeys(Object parameter) {
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
ErrorContext.instance().store();
keyGenerator.processBefore(executor, mappedStatement, null, parameter);
ErrorContext.instance().recall();
}
}它内部维护了核心字段：
- parameterHandler 作用就是将参数替换sql中的占位符的功能
- resultSetHandler 将sql结果集映射成结果对象
- executor 执行sql的抽象，后续详细看
只实现了三个接口方法：
- getBoundSql
- getParameterHandler
- prepare 其中prepare的实现使用了模版模式，子类必须实现
instantiateStatement方法来完成父类prepare方法的模版。
在BaseStatementHandler的构造函数中我们可以看到一段调用generateKeys的代码，它和生成主键key有关。
关于获得主键key和插入数据时放入主键key牵涉到以下几个配置：
- selectKey
- useGeneratedKeys 设置true使用自动生成的主键
- keyProperty 指定主键是(javaBean的)哪个属性。
对于支持自动生成记录主键的数据库，如：MySQL，SQL Server，此时设置useGeneratedKeys参数值为true，在执行添加记录之后可以获取到数据库自动生成的主键ID，合keyProperty指定主键。
例子：
insert into notice(msg_type,title,content,rec_time,send_time,user_id,deleted,viewed)
values(#{msgType,jdbcType=INTEGER},#{title,jdbcType=VARCHAR},#{content,jdbcType=VARCHAR},
#{recTime,jdbcType=BIGINT},#{sendTime,jdbcType=BIGINT},#{userId,jdbcType=VARCHAR},
#{deleted,jdbcType=TINYINT},#{viewed,jdbcType=INTEGER})
如果使用selectKey，可以设置order属性为AFTER。
例子：
SELECT LAST_INSERT_ID()
insert into t_user (username,password,create_date) values(#{username},#{password},#{createDate})
对于Oracle数据库，当要用到自增字段时，需要用到Sequence或者使用外界传入的唯一值比如uuid。则也使用selectKey，设置order为before。
例子：
SELECT SEQ_ADMIN.NEXTVAL FROM DUAL
INSERT INTO tbl_employee(id, name, age, create_time)
VALUES(#{id}, #{name}, #{age}, #{createTime})
KeyGenerator
KeyGenerator接口的实现有：Jdbc3KeyGenerator，SelectKeyGenerator，NoKeyGenerator
useGeneratedKeys设置在settings配置文件中的相关源码在MappedStatement的org.apache.ibatis.mapping.MappedStatement.Builder#Builder中：
mappedStatement.keyGenerator = configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType) ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;默认是NoKeyGenerator，如果配置了useGeneratedKeys=true并且是insert操作则使用Jdbc3KeyGenerator。
useGeneratedKeys设置在mapper文件中的相关源码：
if (configuration.hasKeyGenerator(keyStatementId)) {
keyGenerator = configuration.getKeyGenerator(keyStatementId);
} else {
keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
}如果设置了selectKey则使用SelectKeyGenerator（这部分可以更下去看到），useGeneratedKeys逻辑和前面一样。
selectKey标签中的order分别对应着KeyGenerator中的processBefore方法和processAfter方法。
而前面提到的BaseStatementHandler中generateKeys方法就是触发processBefore的地方，也是唯一一个地方。
protected void generateKeys(Object parameter) {
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
ErrorContext.instance().store();
keyGenerator.processBefore(executor, mappedStatement, null, parameter);
ErrorContext.instance().recall();
}在org.apache.ibatis.executor.statement.StatementHandler#update中的各个实现中都可以看到执行processAfter的代码，比如SimpleStatementHandler的代码：
public int update(Statement statement) throws SQLException {
String sql = boundSql.getSql();
Object parameterObject = boundSql.getParameterObject();
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
int rows;
if (keyGenerator instanceof Jdbc3KeyGenerator) {
statement.execute(sql, Statement.RETURN_GENERATED_KEYS);
rows = statement.getUpdateCount();
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else if (keyGenerator instanceof SelectKeyGenerator) {
statement.execute(sql);
rows = statement.getUpdateCount();
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else {
statement.execute(sql);
rows = statement.getUpdateCount();
}
return rows;
}ParameterHandler
再来看一下前面提到的ParameterHandler，功能就是将动态的sql中的占位符替换成实参。
它的实现是DefaultParameterHandler：
public class DefaultParameterHandler implements ParameterHandler {
private final TypeHandlerRegistry typeHandlerRegistry;
private final MappedStatement mappedStatement;
private final Object parameterObject;
private final BoundSql boundSql;
private final Configuration configuration;
public DefaultParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
this.mappedStatement = mappedStatement;
this.configuration = mappedStatement.getConfiguration();
this.typeHandlerRegistry = mappedStatement.getConfiguration().getTypeHandlerRegistry();
this.parameterObject = parameterObject;
this.boundSql = boundSql;
}
@Override
public Object getParameterObject() {
return parameterObject;
}
@Override
public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
List 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 e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
} catch (SQLException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
}
}
}
}
}
} SimpleStatementHandler类
SimpleStatementHandler是BaseStatementHandler的子类，使用Statement来完成数据库的操作，所以Sql中不会有占位符，parameterize就是空实现。
query方法：
public List query(Statement statement, ResultHandler resultHandler) throws SQLException {
String sql = boundSql.getSql();
statement.execute(sql);
return resultSetHandler.handleResultSets(statement);
} 最后一步将数据库数据转换成java对象，这个后续展开。
update方法：
public int update(Statement statement) throws SQLException {
String sql = boundSql.getSql();
Object parameterObject = boundSql.getParameterObject();
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
int rows;
if (keyGenerator instanceof Jdbc3KeyGenerator) {
statement.execute(sql, Statement.RETURN_GENERATED_KEYS);
rows = statement.getUpdateCount();
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else if (keyGenerator instanceof SelectKeyGenerator) {
statement.execute(sql);
rows = statement.getUpdateCount();
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else {
statement.execute(sql);
rows = statement.getUpdateCount();
}
return rows;
}其中对主键处理的代码前面已经作了解释，从上面两个代码片段来看已经挖到最深了，最终都是调用java.sql.*的api。
PreparedStatementHandler
PreparedStatementHandler使用PreparedStatement实现。
public Cursor queryCursor(Statement statement) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
return resultSetHandler. handleCursorResultSets(ps);
}
public void parameterize(Statement statement) throws SQLException {
parameterHandler.setParameters((PreparedStatement) statement);
} RoutingStatementHandler
看名字就可以猜到了这个是路由用的，看它的构造函数：
private final StatementHandler delegate;
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
switch (ms.getStatementType()) {
case STATEMENT:
delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case PREPARED:
delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case CALLABLE:
delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
default:
throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
}
}所以具体使用哪一个StatementHandler是由MappedStatement.getStatementType()决定的
CallableStatementHandler
依赖CallableStatement，用于调用存储过程。