Add database columns with CURRENT_TIMESTAMP support

[slankka]

New Feature

1. If using DATETIME with current_timestamp, the corresponding data field need not set, it will generated by database.

Robustness Improvement

1. change maven plugin artifact to jpa-entity-generator-maven-plugin. Because calling it maven-<yourplugin>-plugin is an infringement of the Apache Maven Trademark maven plugin development guide
2. forcely write bytes as UTF-8
3. support com.mysql.cj.jdbc.Driver. If not set catalog explicitly, it will get all tables from all databases, and if we get columns without catalog, it will may get duplicated columns from tables (from different catalogs) whose name are same.（Tested in mysql connector 8.0.18).

[slankka]

It takes me some time to find out why the new entity files are missing in sample folder.
finally, Unfortunately I'm not familiar with gradle, 😭 ,and all the files are generated in gradle dist folder, such as

gradle.user.home\daemon\4.10.2\src\main\java\
%USERPROFILE%\.gradle\daemon\4.10.2\src\main\java\

I will commit these files tonight. 😃

update

It only works for gradlew entityGen in sample folder, If I change CodeGenerator:213:

String workingDir = System.getProperty("user.dir");
Path path = Paths.get(workingDir.concat(File.separator) + filepath);

I don't know why if not doing like this, the entity files are generated in gradle dist folders.

[seratch]

Overall, the changes look to me. I left some comments. If other maintainers have some opinions on this PR, please feel free to join the review.

[seratch]

This can be a breaking change. If I remember correctly, setting primitive type here means the value will be false if absent. Is there any reason you changed this?

[slankka]

When the database has a nullable column, primitive type field of the entity can't store null, NPE will occur. Wrapped Type can represent true/false and not set

[slankka]

Assuming we force make all boolean fields to be not null.
There is an another situation:

Sometimes, we use ExampleMatcher of JPA, and telling JPA to ignore the null value field:

ExampleMatcher.matchingAll().withIgnoreNullValues()

or mybatis :

<if test="aNullableBooleanState != null">
a_nullable_boolean_state= #{aNullableBooleanState, jdbcType="BIT"}
</if>

If the field type is primitive, we will always query data with and field =true/false, can't leave this query without this condition.

[seratch]

@slankka
I'm terribly sorry for my belated response here.

That makes sense. That said, I'd like to avoid bringing any breaking changes to this library. This library is widely used at SmartNews and I am no longer working for the company.

If you're still interested in working this PR and have time for it, could you introduce a flag in the configuration allowing users to choose Boolean over the primitive type plus set it as false by default?