java EE Hibernate List many to many configure error - mysql

Question

I am making a java ee web application. I have User and Permission classes and User class have List that types of Permission. When hibernate gets user object form database, i want to get permissions of user from user_permission table ,too. User and Permission has many-to-many relation. I am using Hibernate 5.4.1 . When SessionFactory configurating i get this error:
java.lang.NullPointerException
org.hibernate.boot.model.source.internal.hbm.ModelBinder$AbstractPluralAttributeSecondPass.bindCollectionElement(ModelBinder.java:3557)
org.hibernate.boot.model.source.internal.hbm.ModelBinder$AbstractPluralAttributeSecondPass.doSecondPass(ModelBinder.java:3136)
org.hibernate.boot.internal.InFlightMetadataCollectorImpl.processSecondPasses(InFlightMetadataCollectorImpl.java:1684)
org.hibernate.boot.internal.InFlightMetadataCollectorImpl.processSecondPasses(InFlightMetadataCollectorImpl.java:1652)
org.hibernate.boot.model.process.spi.MetadataBuildingProcess.complete(MetadataBuildingProcess.java:286)
org.hibernate.boot.model.process.spi.MetadataBuildingProcess.build(MetadataBuildingProcess.java:83)
org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:473)
org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:84)
org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:689)
org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:724)
my User class:
public class User
{
private int id;
private String name;
private String password;
private List<Permission> permissions;
public User(){}
public User(String name,String password)
{
this.name=name;
this.password=password;
}
public String getName()
{
return name;
}
public void setName(String name)
{
this.name=name;
}
public String getPassword()
{
return password;
}
public void setPassword(String password)
{
this.password=password;
}
public int getId()
{
return id;
}
public void setId(int id)
{
this.id=id;
}
public void clearPermissions()
{
permissions.clear();
}
public boolean hasPermission(Permission permission)
{
return permissions.contains(permission);
}
public void addPermission(Permission permission)
{
permissions.add(permission);
}
public void deletePermission(Permission permission)
{
permissions.remove(permission);
}
public List<Permission> getPermissions()
{
return permissions;
}
public void setPermissions(List<Permission> permissions)
{
this.permissions = permissions;
}
}
user class mapping :
<?xml version="1.0" encoding="UTF-8"?>
<hibernate-mapping>
<class name="model.User" table="user">
<id name="id" type="int" column="id">
<generator class="identity"/>
</id>
<property name="name" column="name" type="string"/>
<property name="password" column="password" type="string"/>
<list name="permissions" table="user_permission" lazy="false">
<key column="user_id"/>
<index column="id"/>
<many-to-many class="Permission" column="permission_id"/>
</list>
</class>
</hibernate-mapping>
my Permission class:
public class Permission
{
private int id;
private String name;
public Permission() {}
public Permission(int id, String name)
{
this.id=id;
this.name=name;
}
public String getName()
{
return name;
}
public void setName(String name)
{
this.name=name;
}
public int getId()
{
return id;
}
public void setId(int id)
{
this.id=id;
}
public boolean equals(Object obj)
{
return ((Permission)obj).id==id;
}
}
Permission class mapping :
<?xml version="1.0" encoding="UTF-8"?>
<hibernate-mapping>
<class name="model.Permission" table="permission">
<id name="id" type="int" column="id">
<generator class="identity"/>
</id>
<property name="name" column="name" type="string"/>
</class>
</hibernate-mapping>

Answer 1

There is a missing packageName in many to many mapping. Change it from Permission to model.Permission. It should solve the problem. Below is the sippet
<class name="model.User" table="user">
<id name="id" type="int" column="id">
<generator class="identity"/>
</id>
<property name="name" column="name" type="string"/>
<property name="password" column="password" type="string"/>
<list name="permissions" table="user_permission" lazy="false">
<key column="user_id"/>
<index column="id"/>
<many-to-many class="model.Permission" column="permission_id"/>
</list>
</class> </hibernate-mapping>