我尝试运行查询时遇到异常。 这是为什么？

Question

The following snippet throws an exception :

String hqlSelectDB = "use librarian";
sess.createQuery(hqlSelectDB).list();

Exception

java.lang.IllegalArgumentException: node to traverse cannot be null!
org.hibernate.hql.ast.util.NodeTraverser.traverseDepthFirst(NodeTraverser.java:31)
org.hibernate.hql.ast.QueryTranslatorImpl.parse(QueryTranslatorImpl.java:254)
org.hibernate.hql.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:157)
org.hibernate.hql.ast.QueryTranslatorImpl.compile(QueryTranslatorImpl.java:111)
org.hibernate.engine.query.HQLQueryPlan.<init>(HQLQueryPlan.java:77)
org.hibernate.engine.query.HQLQueryPlan.<init>(HQLQueryPlan.java:56)
org.hibernate.engine.query.QueryPlanCache.getHQLQueryPlan(QueryPlanCache.java:72)
org.hibernate.impl.AbstractSessionImpl.getHQLQueryPlan(AbstractSessionImpl.java:133)
org.hibernate.impl.AbstractSessionImpl.createQuery(AbstractSessionImpl.java:112)
org.hibernate.impl.SessionImpl.createQuery(SessionImpl.java:1623)
servlets.InsertPerson.doPost(InsertPerson.java:41)
javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)

Why am I getting this exception ?

Answer 1

use librarian is not a valid query and its not returning any result.

Try some valid query as from librarian.SomeTable t

If you have a look at Hibernate Configuration , you need to specify which DB you want to use, during configuration phase itself.

This is the example of hibernate configuration property file, which will be used by hibernate to make a SessionFactory (A kind of Connection pool), which will give the DB Connection.

hibernate.connection.driver_class = org.postgresql.Driver
hibernate.connection.url = jdbc:postgresql://localhost/mydatabase --> This is where you specify your DB name
hibernate.connection.username = myuser
hibernate.connection.password = secret
hibernate.c3p0.min_size=5
hibernate.c3p0.max_size=20
hibernate.c3p0.timeout=1800
hibernate.c3p0.max_statements=50
hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect

Answer 2

It should be

String hqlSelectDB = "from librarian";// where librarian is data object mapped with DB table
sess.createQuery(hqlSelectDB).list();

Answer 3

Query you are going to use is not a HQL query

From librarian

will work