Resolve POJO constructor arguments by name rather than position

The docs say:

In order to inject the results into the constructor, MyBatis needs to identify the constructor by the type of its parameters. Java has no way to introspect (or reflect) on parameter names. So when creating a constructor element, ensure that the arguments are in order, and that the data types are specified.

But actually there are few ways commonly used to get at the constructor’s parameter names. Spring can wire up immutable beans this way, for example. There’s compiling with debug info (very common nowadays), the older @ConstructorProperties annotation, explicit annotations on each constructor parameter (e.g. MyBatis’s own @Param), and in JDK8 the compilation option -parameters.

It’s a huge advantage, of course, when dealing with immutables to be able to match constructor parameters to fields not positionally, which is quite fragile, but by name.

[harawata]

I have committed the changes. You can specify the parameter name in name attribute of @Arg annotation or <idArg /> and <arg /> elements.

<constructor>
  <arg column="col2" name="secondArg" javaType="string" />
  <arg column="col1" name="firstArg" javaType="int" />
</constructor>

@ConstructorArgs({
  @Arg(column = "col2", name = "secondArg", javaType = String.class),
  @Arg(column = "col1", name = "firstArg", javaType = Integer.class)
})

• To reference constructor parameters by name, you can either 1) annotate parameters with @Param annotation or 2) compile with -parameters option.
• javaType can be omitted if there is a property with the same name and type.
• When MyBatis could not find a matching constructor, it throws an exception during build-phase.

@usethe4ce , @reddyalready It would be great if you guys could try the latest 3.4.3-SNAPSHOT.

[reddyalready]

@harawata Just wanted to thank you again for doing this, I finally got to play around with it - using Kotlin/immutability no less, and it works like a charm 😄

I believe it is not possible yet to create complex immutable objects (resulting from joins, for example) using constructor args, e.g.:

User {
  id: Int = 1,
  address: Address = {
    houseNum: Int = 1,
    line1: String = "Blah Blah"
  }
}

I believe this is related to #101

Can you please confirm?