Three ways to use foreach collection in MyBatis

The main use of foreach is in the build in condition, which can iterate a collection in an SQL statement.

The properties of the Foreach element are mainly item,index,collection,open,separator,close.

The item represents the alias of each element in the collection when it iterates, and index specifies a name that represents the position at which each iteration occurs during the iteration, and open indicates what the statement begins with, and separator indicates what symbol is used as the delimiter between each iteration. Close indicates what ends with.

The most critical and error-prone thing when using foreach is the collection property, which must be specified, but in different cases the value of the property is not the same, there are 3 main cases:

1. If a single parameter is passed in and the parameter type is a list, the collection property value is List
2. If a single parameter is passed in and the parameter type is an array, the value of the collection property is array
3. If the parameters passed in are multiple, we need to encapsulate them into a map, of course, the single parameter can also be encapsulated as a map, in fact, if you pass in the parameter, in the breast will also wrap it into a map, the map key is the parameter name, So this time the collection property value is the key to the incoming list or array object in its own encapsulated map

Here's a look at the sample code for each of the three scenarios:

1. Type of single parameter list:

<select id= "Dynamicforeachtest" parametertype= "java.util.List" resulttype= "Blog" >
	select * from T_blog where ID in
	<foreach collection= "list" index= "index" item= "Item" open= "(" separator= "," close= ")" >
		#{ Item}       
	</foreach>    
</select>

The value of the above collection is list, the corresponding mapper is such a public list dynamicforeachtest (list IDs);
Test code:

@Test public
void Dynamicforeachtest () {
	sqlsession session = Util.getsqlsessionfactory (). Opensession ();
	Blogmapper blogmapper = Session.getmapper (blogmapper.class);
	List ids = new ArrayList ();
	Ids.add (1);
	Ids.add (3);
	Ids.add (6);
	List blogs = blogmapper.dynamicforeachtest (IDS);
	for (blog blog:blogs)
		System.out.println (blog);
	Session.close ();
}

2. Type of single-parameter array arrays:

<select id= "Dynamicforeach2test" parametertype= "java.util.ArrayList" resulttype= "Blog" >
	select * from T_ Blog where ID in
	<foreach collection= "array" index= "index" item= "Item" open= "(" separator= "," close= ")" >
		#{item}
	</foreach>
</select>

The above collection is an array, corresponding to the Mapper code: public List dynamicforeach2test (int[] IDs);
The corresponding test code:

@Test public
void Dynamicforeach2test () {
        sqlsession session = Util.getsqlsessionfactory (). Opensession ();
        Blogmapper blogmapper = Session.getmapper (blogmapper.class);
        int[] ids = new int[] {1,3,6,9};
        List blogs = blogmapper.dynamicforeach2test (IDS);
        for (blog blog:blogs)
        System.out.println (blog);    
        Session.close ();
}

3. Encapsulate the parameters as a map type

<select id= "Dynamicforeach3test" parametertype= "Java.util.HashMap" resulttype= "Blog" >
	select * from T_blog Where title like "%" #{title} "%" and ID in
	<foreach collection= "IDs" index= "index" item= "Item" open= "(" separator= " , "close=") ">
		#{item}
	</foreach>
</select>

The value of the above collection is IDs, which is the key of the incoming parameter Map, corresponding to the Mapper code: public List dynamicforeach3test (Map params);
Corresponding Test code:

@Test public
void Dynamicforeach3test () {
	sqlsession session = Util.getsqlsessionfactory (). Opensession ();
	Blogmapper blogmapper = Session.getmapper (blogmapper.class);
	Final List ids = new ArrayList ();
	Ids.add (1);
	Ids.add (2);
	Ids.add (3);
	Ids.add (6);
	Ids.add (7);
	Ids.add (9);
	MAP params = new HashMap ();
	Params.put ("IDs", IDs);
	Params.put ("title", "China");
	List blogs = blogmapper.dynamicforeach3test (params);
	for (blog blog:blogs)
		System.out.println (blog);
	Session.close ();
}

Reprint Address: https://www.cnblogs.com/fangyu19900812/p/6046209.html

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