Extension private field in java

I read the Spring Data source and meet a question：
JdbcTemplate has a method:

public void setDataSource(@Nullable DataSource dataSource) {
    this.dataSource = dataSource;
}

and I find that this.dataSource is from it`s father class JdbcAccessor.declearing as follow

private DataSource dataSource;

my question is: why extension class JdbcTemplate can access its father class`s private field?
I try to use it as follow and find IDE show its wrong

public abstract class A {
    @Nullable
    private Integer a;
}
public class B extends A {
    public void setA(@Nullable Integer a) {
        this.a = a;    <-- Wrong
    }
}

It can not. Most likely you used some sort of decompiler which couldn't properly decompile it. The actual code is like this in JDBCTemplate

public JdbcTemplate(DataSource dataSource) {
	setDataSource(dataSource);
	afterPropertiesSet();
}

And setDatasource method is in JDBCAccessor.

You cannot access private properties in another class. So you need to add a setter method to add values to private properties.

public abstract class A {
    @Nullable
    private Integer a;
    public void setValue(Integer a){
       this.a=a;
    }
}
public class B extends A {
    public void setA(@Nullable Integer a) {
       this.setValue(a);
    }
}