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8.SQL自动生成插件

本文提供了一种自动生成sql语句的方法,它针对的对象是有主键或唯一索引的单表,提供的操作有增、删、改、查4种。理解本文和本文的提供的代码需要有java注解的知识,因为本文是基于注解生成sql的。

1 准备

1.1 为什么在StatementHandler拦截

在SQL执行流程分析(源码篇)章节介绍了一次sqlsession的完整执行过程,从中可以知道sql的解析是在StatementHandler里完成的,所以为了重写sql需要拦截StatementHandler。

1.2 MetaObject简介

在实现里大量使用了MetaObject这个对象,因此有必要先介绍下它。MetaObject是Mybatis提供的一个的工具类,通过它包装一个对象后可以获取或设置该对象的原本不可访问的属性(比如那些私有属性)。它有个三个重要方法经常用到:
MetaObject forObject(Object object,ObjectFactory objectFactory, ObjectWrapperFactory objectWrapperFactory) 用于包装对象; Object getValue(String name) 用于获取属性的值(支持OGNL的方法); void setValue(String name, Object value) 用于设置属性的值(支持OGNL的方法);

2 拦截器签名

@Intercepts({@Signature(type = StatementHandler.class, method = "prepare", args = {Connection.class})})  
public class AutoMapperInterceptor implements Interceptor {  
    ...  
}
从签名里可以看出,要拦截的目标类型是StatementHandler(注意:type只能配置成接口类型),拦截的方法是名称为prepare参数为Connection类型的方法。

3 intercept实现

@Intercepts({@Signature(type = StatementHandler.class, method = "prepare", args = {Connection.class})})  
public class AutoMapperInterceptor implements Interceptor {  
    private static final Log logger = LogFactory.getLog(AutoMapperInterceptor.class);  
    private static final ObjectFactory DEFAULT_OBJECT_FACTORY = new DefaultObjectFactory();  
    private static final ObjectWrapperFactory DEFAULT_OBJECT_WRAPPER_FACTORY = new DefaultObjectWrapperFactory();  

    @Override  
    public Object intercept(Invocation invocation) throws Throwable {  
        StatementHandler statementHandler = (StatementHandler) invocation.getTarget();  
        MetaObject metaStatementHandler = MetaObject.forObject(statementHandler, DEFAULT_OBJECT_FACTORY,  
                DEFAULT_OBJECT_WRAPPER_FACTORY);  
        // 分离代理对象链  
        while (metaStatementHandler.hasGetter("h")) {  
            Object object = metaStatementHandler.getValue("h");  
            metaStatementHandler = MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);  
        }  
        // 分离最后一个代理对象的目标类  
        while (metaStatementHandler.hasGetter("target")) {  
            Object object = metaStatementHandler.getValue("target");  
            metaStatementHandler = MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);  
        }  
        String originalSql = (String) metaStatementHandler.getValue("delegate.boundSql.sql");  
        Configuration configuration = (Configuration) metaStatementHandler.getValue("delegate.configuration");  
        Object parameterObject = metaStatementHandler.getValue("delegate.boundSql.parameterObject");  
        if (null == originalSql || "".equals(originalSql)) {  
            String newSql = "";  
            MappedStatement mappedStatement = (MappedStatement) metaStatementHandler  
                    .getValue("delegate.mappedStatement");  
            // 根据ID生成相应类型的sql语句(id需剔除namespace信息)  
            String id = mappedStatement.getId();  
            id = id.substring(id.lastIndexOf(".") + 1);  
            if ("insert".equals(id)) {  
                newSql = SqlBuilder.buildInsertSql(parameterObject);  
            } else if ("update".equals(id)) {  
                newSql = SqlBuilder.buildUpdateSql(parameterObject);  
            } else if ("delete".equals(id)) {  
                newSql = SqlBuilder.buildDeleteSql(parameterObject);  
            } else if ("select".equals(id)) {  
                newSql = SqlBuilder.buildSelectSql(parameterObject);  
            }  
            logger.debug("Auto generated sql:" + newSql);  
            //  
            SqlSource sqlSource = buildSqlSource(configuration, newSql, parameterObject.getClass());  
            List<ParameterMapping> parameterMappings = sqlSource.getBoundSql(parameterObject).getParameterMappings();  
            metaStatementHandler.setValue("delegate.boundSql.sql", sqlSource.getBoundSql(parameterObject).getSql());  
            metaStatementHandler.setValue("delegate.boundSql.parameterMappings", parameterMappings);  
        }  
        // 调用原始statementHandler的prepare方法  
        statementHandler = (StatementHandler) metaStatementHandler.getOriginalObject();  
        statementHandler.prepare((Connection) invocation.getArgs()[0]);  
        // 传递给下一个拦截器处理  
        return invocation.proceed();  
    }  

    @Override  
    public Object plugin(Object target) {  
        if (target instanceof StatementHandler) {  
            return Plugin.wrap(target, this);  
        } else {  
            return target;  
        }  
    }  

    @Override  
    public void setProperties(Properties properties) {  

    }  

    private SqlSource buildSqlSource(Configuration configuration, String originalSql,   
    Class<?> parameterType) {  
        SqlSourceBuilder builder = new SqlSourceBuilder(configuration);  
        return builder.parse(originalSql, parameterType, null);  
    }  
}
StatementHandler的默认实现类是RoutingStatementHandler,因此拦截的实际对象是它。RoutingStatementHandler的主要功能是分发,它根据配置Statement类型创建真正执行数据库操作的StatementHandler,并将其保存到delegate属性里。由于delegate是一个私有属性并且没有提供访问它的方法,因此需要借助MetaObject的帮忙。通过MetaObject的封装后我们可以轻易的获得想要的属性。 在上面的方法里有个两个循环,通过他们可以分离出原始的RoutingStatementHandler(而不是代理对象)。 有了插件帮你生成sql语句后,mapper配置文件里单表的增删改查部分就不需要再配置sql代码了,但由于插件需要通过id来生成不同类型的sql语句,因此必要的配置还是需要的,而且相应的id必须是下面的这几个(区分大小写):
<update id="update" parameterType="UserDto"></update>  
<insert id="insert" parameterType="UserDto"></insert>  
<delete id="delete" parameterType="UserDto"></delete>  
<select id="select" parameterType="UserDto" resultType="UserDto""></select>

3 SqlBuilder

SqlBuilder的相应方法接受一个dto对象作为参数,它们根据这个对象的属性值和配置的注解生成相应的sql。
@TableMapperAnnotation(tableName = "t_user", uniqueKeyType = UniqueKeyType.Single, uniqueKey = " userid ")  
public class UserDto {  
    @FieldMapperAnnotation(dbFieldName = "userid", jdbcType = JdbcType.INTEGER)  
    private Integer userid;  
    @FieldMapperAnnotation(dbFieldName = "username", jdbcType = JdbcType.VARCHAR)  
    private String username;  
    ...  
}
这个对象包含了两种注解,一个是TableMapperAnnotation注解,它保存了表名、唯一键类型和构成唯一键的字段;另一个是FieldMapperAnnotation注解,它保存了数据库字段名和字段类型信息。这两个注解都是必须的。SqlBuilder生成sql时会用到他们,下面以生成insert语句的方法为例,其他方法类似:
public static String buildInsertSql(Object object) throws Exception {  
    if (null == object) {  
        throw new RuntimeException("Sorry,I refuse to build sql for a null object!");  
    }  
    Map dtoFieldMap = PropertyUtils.describe(object);  
    // 从参数对象里提取注解信息  
    TableMapper tableMapper = buildTableMapper(object.getClass());  
    // 从表注解里获取表名等信息  
    TableMapperAnnotation tma = (TableMapperAnnotation) tableMapper.getTableMapperAnnotation();  
    String tableName = tma.tableName();  
    StringBuffer tableSql = new StringBuffer();  
    StringBuffer valueSql = new StringBuffer();  

    tableSql.append("insert into ").append(tableName).append("(");  
    valueSql.append("values(");  

    boolean allFieldNull = true;  
    // 根据字段注解和属性值联合生成sql语句  
    for (String dbFieldName : tableMapper.getFieldMapperCache().keySet()) {  
        FieldMapper fieldMapper = tableMapper.getFieldMapperCache().get(dbFieldName);  
        String fieldName = fieldMapper.getFieldName();  
        Object value = dtoFieldMap.get(fieldName);  
        // 由于要根据字段对象值是否为空来判断是否将字段加入到sql语句中,因此DTO对象的属性不能是简单类型,反而必须是封装类型  
        if (value == null) {  
            continue;  
        }  
        allFieldNull = false;  
        tableSql.append(dbFieldName).append(",");  
        valueSql.append("#{").append(fieldName).append(",").append("jdbcType=")  
                .append(fieldMapper.getJdbcType().toString()).append("},");  
    }  
    if (allFieldNull) {  
        throw new RuntimeException("Are you joking? Object " + object.getClass().getName()  
                + "'s all fields are null, how can i build sql for it?!");  
    }  
    tableSql.delete(tableSql.lastIndexOf(","), tableSql.lastIndexOf(",") + 1);  
    valueSql.delete(valueSql.lastIndexOf(","), valueSql.lastIndexOf(",") + 1);  
    return tableSql.append(") ").append(valueSql).append(")").toString();  
}

4 plugin实现

public Object plugin(Object target) {  
    // 当目标类是StatementHandler类型时,才包装目标类,否者直接返回目标本身,减少目标被代理的  
    // 次数  
    if (target instanceof StatementHandler) {  
        return Plugin.wrap(target, this);  
    } else {  
        return target;  
    }  
}
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