MyBatis中添加对exists子查询的支持

2016-12-21 更新
已经加入GitHub托管: https://github.com/leitelyaya/mybatis-3 注意3.2.x
另外已修复ForEach污染全局变量的问题
实际使用效果嘛，临时表数据越少越好，如果不少，请放弃此优化
特感谢这篇博文讲解参数使用：http://blog.csdn.net/isea533/article/details/44002219

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应用系统在设计多功能过滤时往往想到使用多表联结查询，如过滤字段增多，数据所关联的表增大时，数据量大，导致查询时间过长，本着exists优化多表的原则，为Mybatis提供一个方便的编码方式。

E.g. 订单查询列表页，不输入条件时查询所有，输入订单类条件时过滤订单，输入订单收货人、发票、商品信息时，过滤相关的订单
原始SQL：

select * 
from order
left join receiver
using (xx)
left join invoice
using (xx)
left join goods
using (xx)
where 
order...= xx
and receiver... = xx
and invoice... = xx
and goods... = xx

用MyBatis则是：

select * 
from order
left join receiver
using (xx)
left join invoice
using (xx)
left join goods
using (xx)
 ...
<where>
<if test="order...">
and order...= #{order.xx}
</if>
<if test="receiver..."
and receiver...= #{receiver.xx}
</if>
<if test="invoice..."
and invoice...= #{invoice.xx}
</if>
<if test="goods..."
and goods...= #{goods.xx}
</if>
</where>

理想模式下使用exists代替联结优化：

select * 
from order
where 
order...= xx
and exists (
  select 1 from receiver where order.xx=receiver.xx
  and receiver... = xx
)
and exists (
  select 1 from invoice where order.xx=invoice.xx
  and invoice... = xx
)
and exists (
  select 1 from goods where order.xx=goods.xx
  and goods... = xx
)

MyBatis下写得就繁琐了：

select * 
from order
<where> 
<if test="order...">
order...= xx
</if>
<if test="receiver.a!=null && receiver.b!=null && receiver.c!=null ...">
and exists (
  select 1 from receiver where order.xx=receiver.xx
  <if test="receiver.a!=null">
  and receiver.a = #{receiver.a}
  </if>
  <if test="receiver.b!=null">
  and receiver.b = #{receiver.b}
  </if>
  <if test="receiver.c!=null">
  and receiver.c = #{receiver.c}
  </if>
)
...
</where>

当查询条件多的时候，exists外层if的查询就难写了，无奈没有现成检测子查询下所有的if语句的结果返回是不符合的情况

理想处理如下：

select * 
from order
<where> 
<if test="order...">
order...= xx
</if>
<match_any>
and exists (
  select 1 from receiver where order.xx=receiver.xx
  <if test="receiver...">
  and receiver... = xx
  </if>
  <if test="receiver...">
  and receiver... = xx
  </if>
  <if test="receiver...">
  and receiver... = xx
  </if>
)
</match_any>
...
</where>

自定义一个节点，判断上述情况

第一步，更新DTD，使得配置文件验证通过
第二步，配置新解析器，解析自定义节点

新的DTD，在项目资源目录下增加
org.apache.ibatis.builder.xml.mybatis-3-mapper.dtd
注意，项目资源获取优先级大于MyBatis包中的DTD，故而设定为同一个路径，同一个名字，会自动载入改好的文件

<?xml version="1.0" encoding="UTF-8" ?>
<!--

       Copyright 2009-2016 the original author or authors.

       Licensed under the Apache License, Version 2.0 (the "License");
       you may not use this file except in compliance with the License.
       You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

       Unless required by applicable law or agreed to in writing, software
       distributed under the License is distributed on an "AS IS" BASIS,
       WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
       See the License for the specific language governing permissions and
       limitations under the License.

-->
<!ELEMENT mapper (cache-ref | cache | resultMap* | parameterMap* | sql* | insert* | update* | delete* | select* )+>
<!ATTLIST mapper
        namespace CDATA #IMPLIED
        >

<!ELEMENT cache-ref EMPTY>
<!ATTLIST cache-ref
        namespace CDATA #REQUIRED
        >

<!ELEMENT cache (property*)>
<!ATTLIST cache
        type CDATA #IMPLIED
        eviction CDATA #IMPLIED
        flushInterval CDATA #IMPLIED
        size CDATA #IMPLIED
        readOnly CDATA #IMPLIED
        blocking CDATA #IMPLIED
        >

<!ELEMENT parameterMap (parameter+)?>
<!ATTLIST parameterMap
        id CDATA #REQUIRED
        type CDATA #REQUIRED
        >

<!ELEMENT parameter EMPTY>
<!ATTLIST parameter
        property CDATA #REQUIRED
        javaType CDATA #IMPLIED
        jdbcType CDATA #IMPLIED
        mode (IN | OUT | INOUT) #IMPLIED
        resultMap CDATA #IMPLIED
        scale CDATA #IMPLIED
        typeHandler CDATA #IMPLIED
        >

<!ELEMENT resultMap (constructor?,id*,result*,association*,collection*, discriminator?)>
<!ATTLIST resultMap
        id CDATA #REQUIRED
        type CDATA #REQUIRED
        extends CDATA #IMPLIED
        autoMapping (true|false) #IMPLIED
        >

<!ELEMENT constructor (idArg*,arg*)>

<!ELEMENT id EMPTY>
<!ATTLIST id
        property CDATA #IMPLIED
        javaType CDATA #IMPLIED
        column CDATA #IMPLIED
        jdbcType CDATA #IMPLIED
        typeHandler CDATA #IMPLIED
        >

<!ELEMENT result EMPTY>
<!ATTLIST result
        property CDATA #IMPLIED
        javaType CDATA #IMPLIED
        column CDATA #IMPLIED
        jdbcType CDATA #IMPLIED
        typeHandler CDATA #IMPLIED
        >

<!ELEMENT idArg EMPTY>
<!ATTLIST idArg
        javaType CDATA #IMPLIED
        column CDATA #IMPLIED
        jdbcType CDATA #IMPLIED
        typeHandler CDATA #IMPLIED
        select CDATA #IMPLIED
        resultMap CDATA #IMPLIED
        >

<!ELEMENT arg EMPTY>
<!ATTLIST arg
        javaType CDATA #IMPLIED
        column CDATA #IMPLIED
        jdbcType CDATA #IMPLIED
        typeHandler CDATA #IMPLIED
        select CDATA #IMPLIED
        resultMap CDATA #IMPLIED
        >

<!ELEMENT collection (constructor?,id*,result*,association*,collection*, discriminator?)>
<!ATTLIST collection
        property CDATA #REQUIRED
        column CDATA #IMPLIED
        javaType CDATA #IMPLIED
        ofType CDATA #IMPLIED
        jdbcType CDATA #IMPLIED
        select CDATA #IMPLIED
        resultMap CDATA #IMPLIED
        typeHandler CDATA #IMPLIED
        notNullColumn CDATA #IMPLIED
        columnPrefix CDATA #IMPLIED
        resultSet CDATA #IMPLIED
        foreignColumn CDATA #IMPLIED
        autoMapping (true|false) #IMPLIED
        fetchType (lazy|eager) #IMPLIED
        >

<!ELEMENT association (constructor?,id*,result*,association*,collection*, discriminator?)>
<!ATTLIST association
        property CDATA #REQUIRED
        column CDATA #IMPLIED
        javaType CDATA #IMPLIED
        jdbcType CDATA #IMPLIED
        select CDATA #IMPLIED
        resultMap CDATA #IMPLIED
        typeHandler CDATA #IMPLIED
        notNullColumn CDATA #IMPLIED
        columnPrefix CDATA #IMPLIED
        resultSet CDATA #IMPLIED
        foreignColumn CDATA #IMPLIED
        autoMapping (true|false) #IMPLIED
        fetchType (lazy|eager) #IMPLIED
        >

<!ELEMENT discriminator (case+)>
<!ATTLIST discriminator
        column CDATA #IMPLIED
        javaType CDATA #REQUIRED
        jdbcType CDATA #IMPLIED
        typeHandler CDATA #IMPLIED
        >

<!ELEMENT case (constructor?,id*,result*,association*,collection*, discriminator?)>
<!ATTLIST case
        value CDATA #REQUIRED
        resultMap CDATA #IMPLIED
        resultType CDATA #IMPLIED
        >

<!ELEMENT property EMPTY>
<!ATTLIST property
        name CDATA #REQUIRED
        value CDATA #REQUIRED
        >

<!ELEMENT typeAlias EMPTY>
<!ATTLIST typeAlias
        alias CDATA #REQUIRED
        type CDATA #REQUIRED
        >

<!ELEMENT select (#PCDATA | include | trim | where | set | foreach | choose | if | bind | match_any)*>
<!ATTLIST select
        id CDATA #REQUIRED
        parameterMap CDATA #IMPLIED
        parameterType CDATA #IMPLIED
        resultMap CDATA #IMPLIED
        resultType CDATA #IMPLIED
        resultSetType (FORWARD_ONLY | SCROLL_INSENSITIVE | SCROLL_SENSITIVE) #IMPLIED
        statementType (STATEMENT|PREPARED|CALLABLE) #IMPLIED
        fetchSize CDATA #IMPLIED
        timeout CDATA #IMPLIED
        flushCache (true|false) #IMPLIED
        useCache (true|false) #IMPLIED
        databaseId CDATA #IMPLIED
        lang CDATA #IMPLIED
        resultOrdered (true|false) #IMPLIED
        resultSets CDATA #IMPLIED
        >

<!ELEMENT insert (#PCDATA | selectKey | include | trim | where | set | foreach | choose | if | bind | match_any)*>
<!ATTLIST insert
        id CDATA #REQUIRED
        parameterMap CDATA #IMPLIED
        parameterType CDATA #IMPLIED
        timeout CDATA #IMPLIED
        flushCache (true|false) #IMPLIED
        statementType (STATEMENT|PREPARED|CALLABLE) #IMPLIED
        keyProperty CDATA #IMPLIED
        useGeneratedKeys (true|false) #IMPLIED
        keyColumn CDATA #IMPLIED
        databaseId CDATA #IMPLIED
        lang CDATA #IMPLIED
        >

<!ELEMENT selectKey (#PCDATA | include | trim | where | set | foreach | choose | if | bind | match_any)*>
<!ATTLIST selectKey
        resultType CDATA #IMPLIED
        statementType (STATEMENT|PREPARED|CALLABLE) #IMPLIED
        keyProperty CDATA #IMPLIED
        keyColumn CDATA #IMPLIED
        order (BEFORE|AFTER) #IMPLIED
        databaseId CDATA #IMPLIED
        >

<!ELEMENT update (#PCDATA | selectKey | include | trim | where | set | foreach | choose | if | bind | match_any)*>
<!ATTLIST update
        id CDATA #REQUIRED
        parameterMap CDATA #IMPLIED
        parameterType CDATA #IMPLIED
        timeout CDATA #IMPLIED
        flushCache (true|false) #IMPLIED
        statementType (STATEMENT|PREPARED|CALLABLE) #IMPLIED
        keyProperty CDATA #IMPLIED
        useGeneratedKeys (true|false) #IMPLIED
        keyColumn CDATA #IMPLIED
        databaseId CDATA #IMPLIED
        lang CDATA #IMPLIED
        >

<!ELEMENT delete (#PCDATA | include | trim | where | set | foreach | choose | if | bind | match_any)*>
<!ATTLIST delete
        id CDATA #REQUIRED
        parameterMap CDATA #IMPLIED
        parameterType CDATA #IMPLIED
        timeout CDATA #IMPLIED
        flushCache (true|false) #IMPLIED
        statementType (STATEMENT|PREPARED|CALLABLE) #IMPLIED
        databaseId CDATA #IMPLIED
        lang CDATA #IMPLIED
        >

<!-- Dynamic -->

<!ELEMENT include (property+)?>
<!ATTLIST include
        refid CDATA #REQUIRED
        >

<!ELEMENT bind EMPTY>
<!ATTLIST bind
        name CDATA #REQUIRED
        value CDATA #REQUIRED
        >

<!ELEMENT sql (#PCDATA | include | trim | where | set | foreach | choose | if | bind | match_any)*>
<!ATTLIST sql
        id CDATA #REQUIRED
        lang CDATA #IMPLIED
        databaseId CDATA #IMPLIED
        >

<!ELEMENT trim (#PCDATA | include | trim | where | set | foreach | choose | if | bind | match_any)*>
<!ATTLIST trim
        prefix CDATA #IMPLIED
        prefixOverrides CDATA #IMPLIED
        suffix CDATA #IMPLIED
        suffixOverrides CDATA #IMPLIED
        >
<!ELEMENT where (#PCDATA | include | trim | where | set | foreach | choose | if | bind | match_any)*>
<!ELEMENT set (#PCDATA | include | trim | where | set | foreach | choose | if | bind | match_any)*>

<!ELEMENT foreach (#PCDATA | include | trim | where | set | foreach | choose | if | bind | match_any)*>
<!ATTLIST foreach
        collection CDATA #REQUIRED
        item CDATA #IMPLIED
        index CDATA #IMPLIED
        open CDATA #IMPLIED
        close CDATA #IMPLIED
        separator CDATA #IMPLIED
        >

<!ELEMENT choose (when* , otherwise?)>
<!ELEMENT when (#PCDATA | include | trim | where | set | foreach | choose | if | bind | match_any)*>
<!ATTLIST when
        test CDATA #REQUIRED
        >
<!ELEMENT otherwise (#PCDATA | include | trim | where | set | foreach | choose | if | bind | match_any)*>

<!ELEMENT if (#PCDATA | include | trim | where | set | foreach | choose | if | bind | match_any)*>
<!ATTLIST if
        test CDATA #REQUIRED
        >

<!ELEMENT match_any (#PCDATA | include | trim | where | set | foreach | choose | if | bind)*>

更改默认mapper语言解析器：

<settings>
...
<setting name="defaultScriptingLanguage" value="extending"/>
</settings>
<typeAliases>
    <typeAlias type="org.apache.ibatis.scripting.ExtendingXMLLanguageDriver" alias="extending"/>
</typeAliases>

创建解析器：
org.apache.ibatis.scripting.ExtendingXMLLanguageDriver

public class ExtendingXMLLanguageDriver extends XMLLanguageDriver implements LanguageDriver {

    @Override
    public SqlSource createSqlSource(Configuration configuration, XNode script, Class<?> parameterType) {
        return new ExtendingXMLScriptBuilder(configuration, script, parameterType).parseScriptNode();
    }

}

org.apache.ibatis.scripting.xmltags.ExtendingXMLScriptBuilder

public class ExtendingXMLScriptBuilder extends BaseBuilder {

    private XNode context;
    private boolean isDynamic;
    private Class<?> parameterType;

    public ExtendingXMLScriptBuilder(Configuration configuration, XNode context) {
        this(configuration, context, null);
    }

    public ExtendingXMLScriptBuilder(Configuration configuration, XNode context, Class<?> parameterType) {
        super(configuration);
        this.context = context;
        this.parameterType = parameterType;
    }

    public SqlSource parseScriptNode() {
        List<SqlNode> contents = parseDynamicTags(context);
        MixedSqlNode rootSqlNode = new MixedSqlNode(contents);
        SqlSource sqlSource = null;
        if (isDynamic) {
            sqlSource = new DynamicSqlSource(configuration, rootSqlNode);
        } else {
            sqlSource = new RawSqlSource(configuration, rootSqlNode, parameterType);
        }
        return sqlSource;
    }

    private List<SqlNode> parseDynamicTags(XNode node) {
        List<SqlNode> contents = new ArrayList<SqlNode>();
        NodeList children = node.getNode().getChildNodes();
        for (int i = 0; i < children.getLength(); i++) {
            XNode child = node.newXNode(children.item(i));
            if (child.getNode().getNodeType() == Node.CDATA_SECTION_NODE || child.getNode().getNodeType() == Node.TEXT_NODE) {
                String data = child.getStringBody("");
                TextSqlNode textSqlNode = new TextSqlNode(data);
                if (textSqlNode.isDynamic()) {
                    contents.add(textSqlNode);
                    isDynamic = true;
                } else {
                    contents.add(new StaticTextSqlNode(data));
                }
            } else if (child.getNode().getNodeType() == Node.ELEMENT_NODE) { // issue #628
                String nodeName = child.getNode().getNodeName();
                NodeHandler handler = nodeHandlers.get(nodeName);
                if (handler == null) {
                    throw new BuilderException("Unknown element <" + nodeName + "> in SQL statement.");
                }
                handler.handleNode(child, contents);
                isDynamic = true;
            }
        }
        return contents;
    }

    private Map<String, NodeHandler> nodeHandlers = new HashMap<String, NodeHandler>() {
        private static final long serialVersionUID = 7123056019193266281L;

        {
            put("trim", new TrimHandler());
            put("where", new WhereHandler());
            put("set", new SetHandler());
            put("foreach", new ForEachHandler());
            put("if", new IfHandler());
            put("choose", new ChooseHandler());
            put("when", new IfHandler());
            put("otherwise", new OtherwiseHandler());
            put("bind", new BindHandler());
            put("match_any", new MatchAnyHandler());
        }
    };

    private interface NodeHandler {
        void handleNode(XNode nodeToHandle, List<SqlNode> targetContents);
    }

    private class BindHandler implements NodeHandler {
        public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
            final String name = nodeToHandle.getStringAttribute("name");
            final String expression = nodeToHandle.getStringAttribute("value");
            final VarDeclSqlNode node = new VarDeclSqlNode(name, expression);
            targetContents.add(node);
        }
    }

    private class TrimHandler implements NodeHandler {
        public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
            List<SqlNode> contents = parseDynamicTags(nodeToHandle);
            MixedSqlNode mixedSqlNode = new MixedSqlNode(contents);
            String prefix = nodeToHandle.getStringAttribute("prefix");
            String prefixOverrides = nodeToHandle.getStringAttribute("prefixOverrides");
            String suffix = nodeToHandle.getStringAttribute("suffix");
            String suffixOverrides = nodeToHandle.getStringAttribute("suffixOverrides");
            TrimSqlNode trim = new TrimSqlNode(configuration, mixedSqlNode, prefix, prefixOverrides, suffix, suffixOverrides);
            targetContents.add(trim);
        }
    }

    private class WhereHandler implements NodeHandler {
        public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
            List<SqlNode> contents = parseDynamicTags(nodeToHandle);
            MixedSqlNode mixedSqlNode = new MixedSqlNode(contents);
            WhereSqlNode where = new WhereSqlNode(configuration, mixedSqlNode);
            targetContents.add(where);
        }
    }

    private class SetHandler implements NodeHandler {
        public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
            List<SqlNode> contents = parseDynamicTags(nodeToHandle);
            MixedSqlNode mixedSqlNode = new MixedSqlNode(contents);
            SetSqlNode set = new SetSqlNode(configuration, mixedSqlNode);
            targetContents.add(set);
        }
    }

    private class ForEachHandler implements NodeHandler {
        public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
            List<SqlNode> contents = parseDynamicTags(nodeToHandle);
            MixedSqlNode mixedSqlNode = new MixedSqlNode(contents);
            String collection = nodeToHandle.getStringAttribute("collection");
            String item = nodeToHandle.getStringAttribute("item");
            String index = nodeToHandle.getStringAttribute("index");
            String open = nodeToHandle.getStringAttribute("open");
            String close = nodeToHandle.getStringAttribute("close");
            String separator = nodeToHandle.getStringAttribute("separator");
            ForEachSqlNode forEachSqlNode = new ForEachSqlNode(configuration, mixedSqlNode, collection, index, item, open, close, separator);
            targetContents.add(forEachSqlNode);
        }
    }

    private class IfHandler implements NodeHandler {
        public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
            List<SqlNode> contents = parseDynamicTags(nodeToHandle);
            MixedSqlNode mixedSqlNode = new MixedSqlNode(contents);
            String test = nodeToHandle.getStringAttribute("test");
            IfSqlNode ifSqlNode = new IfSqlNode(mixedSqlNode, test);
            targetContents.add(ifSqlNode);
        }
    }

    private class MatchAnyHandler implements NodeHandler {
        public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
            List<SqlNode> contents = parseDynamicTags(nodeToHandle);
            List<SqlNode> dynamicContents = new ArrayList<>();
            for (SqlNode node : contents) {
                if (node instanceof IfSqlNode ||
                        node instanceof ChooseSqlNode ||
                        node instanceof ForEachHandler) {
                    dynamicContents.add(node);
                }
            }
            MixedSqlNode mixedSqlNode = new MixedSqlNode(contents);
            MixedSqlNode dynamicSqlNode = new MixedSqlNode(dynamicContents);
            MatchAnySqlNode matchAnySqlNode = new MatchAnySqlNode(configuration, mixedSqlNode, dynamicSqlNode);
            targetContents.add(matchAnySqlNode);
        }
    }

    private class OtherwiseHandler implements NodeHandler {
        public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
            List<SqlNode> contents = parseDynamicTags(nodeToHandle);
            MixedSqlNode mixedSqlNode = new MixedSqlNode(contents);
            targetContents.add(mixedSqlNode);
        }
    }

    private class ChooseHandler implements NodeHandler {
        public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
            List<SqlNode> whenSqlNodes = new ArrayList<SqlNode>();
            List<SqlNode> otherwiseSqlNodes = new ArrayList<SqlNode>();
            handleWhenOtherwiseNodes(nodeToHandle, whenSqlNodes, otherwiseSqlNodes);
            SqlNode defaultSqlNode = getDefaultSqlNode(otherwiseSqlNodes);
            ChooseSqlNode chooseSqlNode = new ChooseSqlNode(whenSqlNodes, defaultSqlNode);
            targetContents.add(chooseSqlNode);
        }

        private void handleWhenOtherwiseNodes(XNode chooseSqlNode, List<SqlNode> ifSqlNodes, List<SqlNode> defaultSqlNodes) {
            List<XNode> children = chooseSqlNode.getChildren();
            for (XNode child : children) {
                String nodeName = child.getNode().getNodeName();
                NodeHandler handler = nodeHandlers.get(nodeName);
                if (handler instanceof IfHandler) {
                    handler.handleNode(child, ifSqlNodes);
                } else if (handler instanceof OtherwiseHandler) {
                    handler.handleNode(child, defaultSqlNodes);
                }
            }
        }

        private SqlNode getDefaultSqlNode(List<SqlNode> defaultSqlNodes) {
            SqlNode defaultSqlNode = null;
            if (defaultSqlNodes.size() == 1) {
                defaultSqlNode = defaultSqlNodes.get(0);
            } else if (defaultSqlNodes.size() > 1) {
                throw new BuilderException("Too many default (otherwise) elements in choose statement.");
            }
            return defaultSqlNode;
        }
    }

}

添加match_any的标签解析器：
org.apache.ibatis.scripting.xmltags.MatchAnySqlNode

public class MatchAnySqlNode implements SqlNode {
    private final Configuration configuration;
    private final SqlNode dynamicContents;
    private SqlNode contents;

    public MatchAnySqlNode(Configuration configuration, SqlNode contents, SqlNode dynamicContents) {
        this.contents = contents;
        this.dynamicContents = dynamicContents;
        this.configuration = configuration;
    }

    public boolean apply(DynamicContext context) {
        DynamicContext oldContext = context;

        context = new FilterContext(oldContext);
        dynamicContents.apply(context);
        String sqlS = context.getSql();

        context = new FilterContext(oldContext);
        contents.apply(context);
        String sqlN = context.getSql();

        context = oldContext;
        if (sqlS.trim().length() > 0) {
            context.appendSql(sqlN);
            return true;
        }

        return true;
    }

    private class FilterContext extends DynamicContext {
        private DynamicContext delegate;
        StringBuffer buffer;

        public FilterContext(DynamicContext delegate) {
            super(configuration, null);
            buffer = new StringBuffer();
            this.delegate = delegate;
        }

        @Override
        public Map<String, Object> getBindings() {
            return delegate.getBindings();
        }

        @Override
        public void bind(String name, Object value) {
            delegate.bind(name, value);
        }

        @Override
        public void appendSql(String sql) {
            buffer.append(sql);
        }

        @Override
        public String getSql() {
            return buffer.toString();
        }
    }
}

~_~ 简单粗暴搞定..