JPA Overview

JPA State Transistion

State_Overview

 

JPA Method   refresh()

  •  Uses Entity Manager refresh() method when you suspect that your data is changed in your database outside of an JPA operation
  • The Entity itself must be in a managed status or you get IllegalArgumentException
  • Entity Manager refresh() method allows us to revert back changes when a user decides to undo all local Entity operations .
Related JAVA Code 

            tm = (TransactionManager)ctx.lookup("java:/TransactionManager"); 
            setRunTimeInfo("Lookup Transactionmanager: ctx.lookup(java:/TransactionManager)");
            String tx_status_before = returnTXStatus(tm);                  
            tm.begin();
            String tx_status_after = returnTXStatus(tm);
            setRunTimeInfo("Begin Transaction: tm.begin() - Before " + tx_status_before + 
                    " - After " + tx_status_after );
            em=getEntityManager();
            e = em.find(Emp2.class, eno );
            checkEntity(e,em);
           
            Transaction transaction = tm.suspend();     
                // Update via pure JDBC - data is already commited in an autonomous TX!
            setRunTimeInfo(" ------- Update Record via NON-XA datasource - outside from JPA Entity Mangager  ----------");
            doUpdate(e);
            
            setRunTimeInfo(" ------- Retrieving Data after doUpdate()  ----------");
            checkEntity(e,em);
            
            setRunTimeInfo(" Resuming transaction");
            tm.resume(transaction);            
            em.refresh(e);
            setRunTimeInfo(" ------- Retrieving Data after em.refesh() ----------");
            checkEntity(e,em);
                 
            tm.rollback();
            setRunTimeInfo("After Resuming/rollback  transaction");
            checkEntity(e,em);
            setRunTimeInfo("The next em.refresh(e) should throw an execption as the Entity is not managed anymore");
            em.refresh(e);

Related Output:
11:16:07.453 Calling refreshEntity()in progress - ID: 9997 - useJoinTransaction : true
11:16:07.454  ------- Refresh test ----------
11:16:07.454 Lookup Transactionmanager: ctx.lookup(java:/TransactionManager)
11:16:07.457 Begin Transaction: tm.begin() - Before [TM status:  6 - STATUS_NO_TRANSACTION] - After [TM status:  0 - STATUS_ACTIVE]
11:16:10.154 checkEntity() -  :  empno: 9997 - Sal: 2800 - Entity Salary Persistence : 2800 - Managed Entity Status : true - STALE Data: false
11:16:10.154  ------- Update Record via NON-XA datasource - outside from JPA Entity Mangager  ---------
11:16:10.212 Leaving doUpdate() without Exceptions - Data commited !
11:16:10.212  ------- Retrieving Data after doUpdate()  ----------
11:16:10.214 checkEntity() -  :  empno: 9997 - Sal: 2900 - Entity Salary Persistence : 2800 - Managed Entity Status : true - STALE Data: true
11:16:10.214  Resuming transaction
11:16:10.216  ------- Retrieving Data after em.refesh() ----------
11:16:10.217 checkEntity() -  :  empno: 9997 - Sal: 2900 - Entity Salary Persistence : 2900 - Managed Entity Status : true - STALE Data: false
11:16:10.218 After Resuming/rollback  transaction
11:16:10.219 checkEntity() -  :  empno: 9997 - Sal: 2900 - Entity Salary Persistence : 2900 - Managed Entity Status : false - STALE Data: false
11:16:10.220 The next em.refresh(e) should throw an execption as the Entity is not managed anymore
11:16:10.220 FATAL ERROR in  refreshEntity(): 
11:16:10.222 Potential Root Cause : Can not refresh not managed object: com.hhu.wfjpa2el.Emp2[ empno=9997 ].
11:16:10.223 Leaving refreshEntity() - Entity manager  closed !

Related Exception:
11:16:10.220 Error in top level function: refreshEntity()
11:16:10.221 Can not refresh not managed object: com.hhu.wfjpa2el.Emp2[ empno=9997 ].
11:16:10.222 java.lang.IllegalArgumentException: Can not refresh not managed object: com.hhu.wfjpa2el.Emp2[ empno=9997 ].
    at org.eclipse.persistence.internal.jpa.EntityManagerImpl.refresh(EntityManagerImpl.java:1024)
    at org.eclipse.persistence.internal.jpa.EntityManagerImpl.refresh(EntityManagerImpl.java:929)
    at com.hhu.wfjpa2el.JPATestBean.refreshEntity(JPATestBean.java:661)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

Using Cache

There are two level of cache: L1, L2

  • L1 is the Entitymanager Persistance Context.
  • L2 is the EMF context .It can be shared with all Enitymanagers in the application

Cache Retrieval Mode
The cache retrieval mode, set by the javax.persistence.retrieveMode property, controls how data is
read from the cache for calls to the EntityManager.find method and from queries.
The retrieveMode property can be set to one of the constants defined by the 
javax.persistence.CacheRetrieveMode enumerated type, either USE (the default) or BYPASS. When it is
set to USE, data is retrieved from the second-level cache, if available. If the data is not in the 
cache, the persistence provider will read it from the database. 
When it is set to BYPASS, the second-level cache is bypassed and a call to the database is made to 
retrieve the data.

Cache Store Mode
The cache store mode, set by the javax.persistence.storeMode property, controls how data is stored 
in the cache. The storeMode property can be set to one of the constants defined by the 
javax.persistence.CacheStoreMode enumerated type, either USE (the default), BYPASS, or REFRESH. 
When set to USE the cache data is created or updated when data is read from or committed to the 
database. If data is already in the cache, setting the store mode to USE will not force 
a refresh when data is read from the database.

When the store mode is set to BYPASS, data read from or committed to the database is not inserted 
or updated in the cache. That is, the cache is unchanged.
When the store mode is set to REFRESH, the cache data is created or updated when data is read 
from or committed to the database, and a refresh is forced on data in the cache upon database 
reads.

JPA Cache Details 
Enum CacheRetrieveMode
  Used as the value of the javax.persistence.cache.retrieveMode property to specify the behavior 
  when  data is retrieved by the find methods and by queries. 
CacheRetrieveMode BYPASS
    Bypass the cache: get data directly from the database. 
    Since:    JPA 2.0
CacheRetrieveMode USE
    Read entity data from the cache: this is the default behavior.
    Since:    JPA 2.0

Enum CacheStoreMode
   Used as the value of the javax.persistence.cache.storeMode property to specify the behavior when 
   data is read from the database and when data is committed into the database.
CacheStoreMode BYPASS
    Don't insert into cache.
    Since:  JPA 2.0
CacheStoreMode REFRESH
    Insert/update entity data into cache when read from database and when committed into database. 
    Forces refresh of cache for items read from database.
    Since:  JPA 2.0
CacheStoreMode USE
    Insert/update entity data into cache when read from database and when committed into database: 
    this is the default behavior. 
    Does not force refresh of already cached items when reading from database.
    Since:  JPA 2.0

JPA and Object Comparison

  • Note the Netbeans ORM tool creates code only  to compare the PK field.
    You need to extend the equals function to cover add. fields  .

Sample adding the sal field for object comparison:
public class Emp2 implements Serializable
    {
    private static final long serialVersionUID = 1L;
    @Id
    @Basic(optional = false)
    @NotNull
    @Column(name = "EMPNO"
...
@Override
    public boolean equals(Object object)
      {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof Emp2))
          {
            return false;
          }
        Emp2 other = (Emp2) object;
        if ((this.empno == null && other.empno != null) || (this.empno != null && !this.empno.equals(other.empno)))
          {
            return false;
          }
        // Newly added to extend equals method to check sal fied too.
        if ((this.sal == null && other.sal != null) || (this.sal != null && !this.sal.equals(other.sal)))
          {
            return false;
          }
        return true;
      }

Using JTA and Entity Manager [ see Pro JPA2 – page 129 ]

-  If the Persistence Manager is created outside the transaction ( before ut.begin() ) we need to join 
   the transaction using by using  em.joinTransaction();         
-  Without joining you very likely see following error:
    09:38:33.272 Cannot call methods requiring a transaction if the entity manager has not been joined to the current transaction.
    09:38:33.272 javax.persistence.TransactionRequiredException: Cannot call methods requiring a transaction if the 
                 entity manager has not been joined to the current transaction.
        at org.eclipse.persistence.internal.jpa.EntityManagerImpl.checkForTransaction(EntityManagerImpl.java:2045)
        at org.eclipse.persistence.internal.jpa.EntityManagerImpl.flush(EntityManagerImpl.java:863)
        at com.hhu.wfjpa2el.JPATestBean.runJPA(JPATestBean.java:201)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  
 Current Code: 
      em=getEntityManager();
      ut.begin();               
      em.joinTransaction();           
      em.persist(e);            
      em.flush();           
      ut.commit();
  
  Workaround 1 : Create the Entity Manager after ut.begin()
      ut.begin();  
      em=getEntityManager();                       
      em.persist(e);            
      em.flush();           
      ut.commit();
  
  Workaround 2: Use  em.joinTransaction()
      em=getEntityManager();
      ut.begin();               
      em.joinTransaction();           
      em.persist(e);            
      em.flush();           
      ut.commit();

JPA and Anotations

JAXP Anotations

@XmlRootElement                        Define the root element for the XML to be produced with @XmlRootElement JAXB annotation
@XmlAccessorType(XmlAccessType.FIELD)  Defined at  class level. Annotate private fields instead of getter methods.
@XmlTransient                          Annotate fields that we do not want to be included in XML or JSON output with @XMLTransient.

JPA Anotations

@Entity                                 annotation defines that a class can be mapped to a table
@Id                                     Marks a field as a Primary Key. Requirement
@GeneratedValue(strategy = GenerationType.TABLE, generator = "accountSequence") Use a sequence to AUTO generate Primary Keys

Anotation Sample 

@Entity
@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
@Table(name = "Account")
public class Account extends AbstractBaseEntity {

    public Account() {
        publishers = new LinkedList<>();
    }
    /**
     * Primary key.
     */
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "accountSequence")
    @TableGenerator(name = "accountSequence")
    @XmlTransient
    private Long id;

    /**
     * The login name of the user.
     * Is different form user real name. Normally this value is taken from external
     * authentication system e.g. LDAP.
     */
    @NotNull
    @Column(nullable = false, length = 45, unique = true)
    private String loginName;
  • Field id is anotated with @XmlTransient and will not be included in the XML / JSON Outout
  • Field loginName is NOT anotated with @XmlTransient and bnot be included in the XML / JSON Outout

Reference

JPA @OneToManyRelationshiip

JPA System errors: javax.ejb.EJBTransactionRolledbackException

Error

[2018-11-07T14:22:58.273+0100] [Payara 5.182] [WARNING] [AS-EJB-00056] [javax.enterprise.ejb.container] [tid: _ThreadID=38 _
 ThreadName=http-thread-pool::http-listener-1(4)] [timeMillis: 1541596978273] [levelValue: 900] [[
  A system exception occurred during an invocation on EJB SemesterService, method: public int de.thnuernberg.in.stuv.pythia.app.services.SemesterService.retrieveSemesterData(int)]]

[2018-11-07T14:22:58.273+0100] [Payara 5.182] [WARNING] [] [javax.enterprise.ejb.container] [tid: _ThreadID=38 _
ThreadName=http-thread-pool::http-listener-1(4)] [timeMillis: 1541596978273] [levelValue: 900] [[
  
javax.ejb.EJBTransactionRolledbackException
	at com.sun.ejb.containers.BaseContainer.mapLocal3xException(BaseContainer.java:2390)
	at com.sun.ejb.containers.BaseContainer.postInvoke(BaseContainer.java:2172)
	at com.sun.ejb.containers.BaseContainer.postInvoke(BaseContainer.java:2093)

Explanation

Any exception that is a subclass of Exception, but not a subclass of RuntimeException and RemoteException, is an application exception. 
All the application exceptions are checked exceptions, So, when we call another method in our method that can throw an application exception, 
we must either declare it in the throws clause of the calling method or catch it in the body of the calling method or both.

All system exceptions are unchecked exceptions except RemoteExceptions and it can't be handled by the user.

Root Cause

  • Drop Table command when the JTA Datasource was still connected !
  • Note int this cause data from Entity Manager and Database are not in sync anymore
  • A not catchable System Execption is thrown and only a Webserver reboot will fix the db/entity manager integrity

Fix

  • Restart Payara !

Reference

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