mybatis动态sql实现逻辑代码详解

mybatis通过将sql配置xml文件中,通过解析xml动态标签来实现动态sql
如下样例 xml文件

<?xml version = "1.0" ?>
<!doctype script system "script-1.0.dtd">
<script namespace="user">
    <common id="commonorder">
        order by id desc
    </common>
    <sql id="queryuser">
        select * from user
        <where>
            <if test='id != null '>
                id = #{id}
            </if>
            <if test="names != null and names.size() >0">
                and name in
                <foreach collection="names" item="item" separator="," open="(" close=")">
                    ${item}
                </foreach>
            </if>
        </where>
        <ref id="commonorder"/>
    </sql>

    <sql id="updateuser">
        update user set name = ${name}
        <where>
            <if test='id != null '>
                id = #{id}
            </if>
        </where>
    </sql>

 
</script>

1.xml文件读取

xml标签编写规则

<!element script (#pcdata | sql | common)*>
<!attlist script
namespace cdata #required
>
<!element sql (#pcdata | trim | where | set | foreach | choose | if | ref)*>
<!attlist sql
id cdata #required
>
<!element common (#pcdata | trim | where | set | foreach | choose | if)*>
<!attlist common
id cdata #required
>
<!element ref (#pcdata)*>
<!attlist ref
id cdata #required
>
<!element trim (#pcdata | trim | where | set | foreach | choose | if)*>
<!attlist trim
prefix cdata #implied
prefixoverrides cdata #implied
suffix cdata #implied
suffixoverrides cdata #implied
>
<!element where (#pcdata | trim | where | set | foreach | choose | if | ref)*>
<!element set (#pcdata | trim | where | set | foreach | choose | if)*>

<!element foreach (#pcdata | trim | where | set | foreach | choose | if)*>
<!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 | trim | where | set | foreach | choose | if)*>
<!attlist when
test cdata #required
>
<!element otherwise (#pcdata | trim | where | set | foreach | choose | if)*>

<!element if (#pcdata | trim | where | set | foreach | choose | if)*>
<!attlist if
test cdata #required
>

documentbuilderfactory 是jdk自带的解析xml文件操作,位于 javax.xml.parsers 包,如图其是一个抽象类,因此无法被实例化

需要通过 newinstance 来实例化

实例化对象后的属性 setvalidating(true); 即通过xml的 验证规则进行xml格式验证

setnamespaceaware 设置忽略命名空间
对于xml文件的命名空间的定义,详见

private document buildxml(inputstream scriptfile)
				throws parserconfigurationexception, saxexception, ioexception {
			documentbuilderfactory factory = documentbuilderfactory
					.newinstance();
			//默认情况下，解析器不验证文档。将这个参数设置为 true 可打开验证功能。
			factory.setvalidating(true);
			//是否设置命名空间
			factory.setnamespaceaware(false);
			//确定是否要忽略文件中的注释。其默认值为 false。
			factory.setignoringcomments(true);
			//确定是否要忽略元素内容中的空白（类似于浏览器对待 html 的方式）。其默认值为 false。
			factory.setignoringelementcontentwhitespace(false);
			//定解析器是否要将 cdata 节点转换为文本，以及是否要和周围的文本节点合并（如果适用的话）。其默认值为 false。
			factory.setcoalescing(false);
			//确定是否要展开外部实体引用。如果为 true，外部数据将插入文档。其默认值为 true
			factory.setexpandentityreferences(true);

			documentbuilder builder = factory.newdocumentbuilder();
			//设置验证规则文件 dtd文件
			builder.setentityresolver(new entityresolver() {
				@override
				public inputsource resolveentity(string publicid,
						string systemid) throws saxexception, ioexception {
					return new inputsource(new classpathresource("script-1.0.dtd").getinputstream());
				}
			});
			//设置错误解析器
			builder.seterrorhandler(new errorhandler() {
				@override
				public void error(saxparseexception exception)
						throws saxexception {
					throw exception;
				}

				@override
				public void fatalerror(saxparseexception exception)
						throws saxexception {
					throw exception;
				}

				@override
				public void warning(saxparseexception exception)
						throws saxexception {
				}
			});
			return builder.parse(scriptfile);
		}

2.xml 文件解析

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