StatementHandler接口是MyBatis的核心接口之一,它完成了MyBatis中最核心的工作,也是Executor 接口實現的基礎。
StatementHandler接口中的功能很多,例如創建Statement對象,爲SQL語句綁定實參,執行select、insert、update、delete等多種類型的SQL語句,批量執行SQL語句,將結果集映射成結果對象。
public interface StatementHandler {
// 從連接中獲取一個Statement
Statement prepare(Connection connection, Integer transactionTimeout)
throws SQLException;
// 綁定statement執行時所需的實參
void parameterize(Statement statement)
throws SQLException;
// 批量執行SQL語句
void batch(Statement statement)
throws SQLException;
// 執行update/insert/delete語句
int update(Statement statement)
throws SQLException;
// 執行select語句
<E> List<E> query(Statement statement, ResultHandler resultHandler)
throws SQLException;
<E> Cursor<E> queryCursor(Statement statement)
throws SQLException;
BoundSql getBoundSql();
// 獲取參數處理器
ParameterHandler getParameterHandler();
}
RoutingStatementHandler
RoutingStatementHandler使用了策略模式,RoutingStatementHandler是策略類,而SimpleStatementHandler、PreparedStatementHandler、CallableStatementHandler則是實現了具體算法的實現類,RoutingStatementHandler對象會根據MappedStatement對象的StatementType屬性值選擇使用相應的策略去執行。
public class RoutingStatementHandler implements StatementHandler {
// 持有的真正實現StatementHandler接口功能的對象
private final StatementHandler delegate;
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
// RoutingStatementHandler的作用就是根據ms的配置,生成一個相對應的StatementHandler對象
// 並設置到持有的delegate屬性中,本對象的所有方法都是通過調用delegate的相應方法實現的
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());
}
}
@Override
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
return delegate.prepare(connection, transactionTimeout);
}
@Override
public void parameterize(Statement statement) throws SQLException {
delegate.parameterize(statement);
}
@Override
public void batch(Statement statement) throws SQLException {
delegate.batch(statement);
}
@Override
public int update(Statement statement) throws SQLException {
return delegate.update(statement);
}
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
return delegate.query(statement, resultHandler);
}
@Override
public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
return delegate.queryCursor(statement);
}
@Override
public BoundSql getBoundSql() {
return delegate.getBoundSql();
}
@Override
public ParameterHandler getParameterHandler() {
return delegate.getParameterHandler();
}
}
BaseStatementHandler
看它以Base開頭,就可以猜到 它是一個實現了StatementHandler接口的抽象類,這個類只提供了一些參數綁定相關的方法,並沒有實現操作數據庫的方法。
public abstract class BaseStatementHandler implements StatementHandler {
// 持有的這些屬性都是通過構造方法完成初始化的,typeHandlerRegistry、
// objectFactory、parameterHandler等則是通過configuration屬性獲得的
protected final Configuration configuration;
protected final ObjectFactory objectFactory;
protected final TypeHandlerRegistry typeHandlerRegistry;
protected final ResultSetHandler resultSetHandler;
// parameterHandler的功能主要是爲SQL語句綁定實參,也就是使用傳入的實參
// 替換SQL語句中的佔位符"?"
protected final ParameterHandler parameterHandler;
// 用來執行SQL語句的執行器
protected final Executor executor;
protected final MappedStatement mappedStatement;
// 記錄了用戶設置的offset和limit,用於在結果集中定位
// 映射的起始位置和結束位置
protected final RowBounds rowBounds;
protected BoundSql boundSql;
// BaseStatementHandler的構造方法主要用於屬性的初始化
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
// 其中調用了KeyGenerator的processBefore()方法
// 用於初始化SQL語句的主鍵
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);
}
protected void generateKeys(Object parameter) {
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
ErrorContext.instance().store();
keyGenerator.processBefore(executor, mappedStatement, null, parameter);
ErrorContext.instance().recall();
}
@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 {
// 這是一個抽象方法,用於初始化java.sql.Statement對象
statement = instantiateStatement(connection);
// 爲Statement對象設置超時時間及fetchSize
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
}
}
}
BaseStatementHandler主要實現了StatementHandler接口中的prepare()方法,BaseStatementHandler依賴兩個重要的組件,ParameterHandler和ResultSetHandler。
ParameterHandler系列組件
我們要執行的SQL語句中可能包含佔位符"?",而每個"?"都對應了BoundSql中parameterMappings集合中的一個元素,在該ParameterMapping對象中記錄了對應的參數名稱以及該參數的相關屬性。ParameterHandler接口定義了一個非常重要的方法setParameters(),該方法主要負責調用PreparedStatement的set*()系列方法,爲SQL語句綁定實參。MyBatis只爲ParameterHandler接口提供了唯一一個實現類DefaultParameterHandler。
public interface ParameterHandler {
// 獲取用戶傳入的實參對象
Object getParameterObject();
// 本方法主要負責調用PreparedStatement.set*()方法,爲SQL語句綁定實參。
void setParameters(PreparedStatement ps)
throws SQLException;
}
public class DefaultParameterHandler implements ParameterHandler {
// 管理mybatis中所有的TypeHandler對象
private final TypeHandlerRegistry typeHandlerRegistry;
// 其中記錄了SQL節點相應的配置信息
private final MappedStatement mappedStatement;
// 用戶傳入的實參對象
private final Object parameterObject;
// 其中記錄了要執行的SQL語句,及參數信息
private final BoundSql boundSql;
private final Configuration configuration;
// 構造方法主要爲持有的屬性 進行初始化
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;
}
// 爲PreparedStatement對象要執行的SQL語句中的佔位符 設置對應的參數值
@Override
public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
// 獲取參數列表
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings != null) {
for (int i = 0; i < parameterMappings.size(); i++) {
ParameterMapping parameterMapping = parameterMappings.get(i);
// 過濾掉存儲過程中的輸出參數
if (parameterMapping.getMode() != ParameterMode.OUT) {
// 記錄綁定的實參
Object value;
// 獲取參數對應的屬性名
String propertyName = parameterMapping.getProperty();
// 根據屬性名 獲取 實參值
if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
value = boundSql.getAdditionalParameter(propertyName);
// 整個實參爲空
} else if (parameterObject == null) {
value = null;
// 如果實參可以直接通過TypeHandler轉換成JdbcType
} else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
value = parameterObject;
} else {
// 獲取對象中相應的屬性值 或查找Map對象中的值
MetaObject metaObject = configuration.newMetaObject(parameterObject);
value = metaObject.getValue(propertyName);
}
// 獲取當前parameterMapping中的TypeHandler對象 及JdbcType對象
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if (value == null && jdbcType == null) {
jdbcType = configuration.getJdbcTypeForNull();
}
try {
// TypeHandler的setParameter()方法會調用PreparedStatement對象的
// set*()系列方法,爲SQL語句綁定相應的實參
typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException | SQLException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
}
}
}
}
}
}
爲SQL語句綁定完實參之後,就可以調用Statement對象 相應的execute方法,將SQL語句交給數據庫執行了。
SimpleStatementHandler
SimpleStatementHandler繼承了BaseStatementHandler抽象類。其底層使用java.sql.Statement來完成數據庫的相關操作,所以SQL語句中不存在佔位符,所以SimpleStatementHandler的parameterize()方法是空實現。SimpleStatementHandler的instantiateStatement()方法直接通過JDBC Connection創建Statement對象。
public class SimpleStatementHandler extends BaseStatementHandler {
// 構造方法主要用於屬性的初始化
public SimpleStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
}
// 直接通過Connection創建Statement對象
@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
if (mappedStatement.getResultSetType() == ResultSetType.DEFAULT) {
// 如果結果集類型是DEFAULT默認的,則直接用connection創建Statement對象
return connection.createStatement();
} else {
// 否則,設置結果集類型,設置結果集 只讀
return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
}
}
// 上面創建的Statement對象會被本方法用於完成數據庫查詢操作
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
// 獲取SQL語句
String sql = boundSql.getSql();
// 發送請求 執行SQL語句
statement.execute(sql);
// 從statement中獲取結果集,並進行映射處理
return resultSetHandler.handleResultSets(statement);
}
// 下面的batch()及queryCursor()方法的實現與上面的query()方法非常類似
@Override
public void batch(Statement statement) throws SQLException {
String sql = boundSql.getSql();
statement.addBatch(sql);
}
@Override
public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
String sql = boundSql.getSql();
statement.execute(sql);
return resultSetHandler.handleCursorResultSets(statement);
}
// 本方法用於執行insert、delete、update等類型的SQL語句,並且會根據配置的
// KeyGenerator獲取數據庫生成的主鍵
@Override
public int update(Statement statement) throws SQLException {
// 獲取SQL語句 及parameterObject
String sql = boundSql.getSql();
Object parameterObject = boundSql.getParameterObject();
// 獲取配置的KeyGenerator 數據庫主鍵生成器
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
int rows;
if (keyGenerator instanceof Jdbc3KeyGenerator) {
// 執行SQL語句
statement.execute(sql, Statement.RETURN_GENERATED_KEYS);
// 獲取更新的條數
rows = statement.getUpdateCount();
// 將數據庫生成的主鍵添加到parameterObject中
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else if (keyGenerator instanceof SelectKeyGenerator) {
// 執行SQL語句
statement.execute(sql);
// 獲取更新的條數
rows = statement.getUpdateCount();
// 執行<selectKey>節點中配置的SQL語句,將從數據庫獲取到的主鍵 添加到parameterObject中
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else {
statement.execute(sql);
rows = statement.getUpdateCount();
}
return rows;
}
@Override
public void parameterize(Statement statement) {
// N/A
}
}
PreparedStatementHandler
PreparedStatementHandler底層依賴於java.sql.PreparedStatement來完成數據庫的相關操作。其中的parameterize()方法中,會調用前面介紹的ParameterHandler的setParameters()方法 完成 SQL語句的參數綁定。instantiateStatement()方法直接調用JDBC Connection的prepareStatement()方法創建PreparedStatement對象。
public class PreparedStatementHandler extends BaseStatementHandler {
// 構造方法主要用於屬性的初始化
public PreparedStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
}
@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
// 獲取SQL語句
String sql = boundSql.getSql();
// 根據mappedStatement持有的KeyGenerator的類型進行不同的處理
if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
// 獲取主鍵列
String[] keyColumnNames = mappedStatement.getKeyColumns();
if (keyColumnNames == null) {
// 返回數據庫生成的主鍵
return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
} else {
// 在insert語句執行完後,會將keyColumnNames指定的列返回
return connection.prepareStatement(sql, keyColumnNames);
}
} else if (mappedStatement.getResultSetType() == ResultSetType.DEFAULT) {
// 如果結果集類型是DEFAULT默認的,則直接通過connection獲取PreparedStatement對象
return connection.prepareStatement(sql);
} else {
// 否則,設置結果集類型,設置結果集爲只讀
return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
}
}
// 因爲是PrepareStatement對象,所以需要處理佔位符"?"
// 使用了前面介紹的ParameterHandler組件完成
@Override
public void parameterize(Statement statement) throws SQLException {
parameterHandler.setParameters((PreparedStatement) statement);
}
// 下面的這些方法,除了多了一步 將Statement對象強轉成PreparedStatement對象
// 其它的幾乎與SimpleStatementHandler一樣
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
return resultSetHandler.handleResultSets(ps);
}
@Override
public void batch(Statement statement) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.addBatch();
}
@Override
public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
return resultSetHandler.handleCursorResultSets(ps);
}
@Override
public int update(Statement statement) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
int rows = ps.getUpdateCount();
Object parameterObject = boundSql.getParameterObject();
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject);
return rows;
}
}
另外,StatementHandler接口還有一個CallableStatementHandler的實現。其底層依賴於java.sql.CallableStatement調用指定的存儲過程,其parameterize()方法也會調用ParameterHandler的setParameters()方法完成SQL語句的參數綁定,並指定輸出參數的索引位置和JDBC類型。其餘方法與前面介紹的ResultSetHandler實現類似,唯一區別是會調用ResultSetHandler的handleOutputParameters()方法 處理輸出參數。
看到這裏,我們可以發現StatementHandler組件依賴ParameterHandler組件 和 ResultSetHandler組件 完成了MyBatis的核心功能,它控制着參數綁定、SQL語句執行、結果集映射等一系列核心流程。