JPA Query

1. JPQL API

The javax.persistence.Query interface is the mechanism for issuing queries in JPA. The primary query language used is the Java Persistence Query Language, or JPQL. JPQL is syntactically very similar to SQL, but is object-oriented rather than table-oriented.

The API for executing JPQL queries will be discussed in Section 1, “ JPQL API ”, and a full language reference will be covered in Section 2, “ JPQL Language Reference ”.

1.  JPQL API

1.1.  Query Basics

SELECT x FROM Magazine x

The preceding is a simple JPQL query for all Magazine entities.

public Query createQuery(String jpql);

The EntityManager.createQuery method creates a Query instance from a given JPQL string.

public List getResultList();

Invoking Query.getResultList executes the query and returns a List containing the matching objects. The following example executes our Magazine query above:

EntityManager em = ...
Query q = em.createQuery("SELECT x FROM Magazine x");
List<Magazine> results = (List<Magazine>) q.getResultList();

A JPQL query has an internal namespace declared in the from clause of the query. Arbitrary identifiers are assigned to entities so that they can be referenced elsewhere in the query. In the query example above, the identifier x is assigned to the entity Magazine .

Note

The as keyword can optionally be used when declaring identifiers in the from clause. SELECT x FROM Magazine x and SELECT x FROM Magazine AS x are synonymous.

Following the select clause of the query is the object or objects that the query returns. In the case of the query above, the query's result list will contain instances of the Magazine class.

Note

When selecting entities, you can optional use the keyword object . The clauses select x and SELECT OBJECT(x) are synonymous.

The optional where clause places criteria on matching results. For example:

SELECT x FROM Magazine x WHERE x.title = 'JDJ'

Keywords in JPQL expressions are case-insensitive, but entity, identifier, and member names are not. For example, the expression above could also be expressed as:

SELECT x FROM Magazine x WHERE x.title = 'JDJ'

But it could not be expressed as:

SELECT x FROM Magazine x WHERE x.TITLE = 'JDJ'

As with the select clause, alias names in the where clause are resolved to the entity declared in the from clause. The query above could be described in English as "for all Magazine instances x, return a list of every x such that x's titlefield is equal to 'JDJ'".

JPQL uses SQL-like syntax for query criteria. The and and or logical operators chain multiple criteria together:

SELECT x FROM Magazine x WHERE x.title = 'JDJ' OR x.title = 'JavaPro'

The = operator tests for equality. <> tests for inequality. JPQL also supports the following arithmetic operators for numeric comparisons: >, >=, <, <=. For example:

SELECT x FROM Magazine x WHERE x.price > 3.00 AND x.price <= 5.00

This query returns all magazines whose price is greater than 3.00 and less than or equal to 5.00.

SELECT x FROM Magazine x WHERE x.price <> 3.00

This query returns all Magazines whose price is not equals to 3.00.

You can group expressions together using parentheses in order to specify how they are evaluated. This is similar to how parentheses are used in Java. For example:

SELECT x FROM Magazine x WHERE (x.price > 3.00 AND x.price <= 5.00) OR x.price = 7.00

This expression would match magazines whose price is 4.00, 5.00, or 7.00, but not 6.00. Alternately:

SELECT x FROM Magazine x WHERE x.price > 3.00 AND (x.price <= 5.00 OR x.price = 7.00)

This expression will magazines whose price is 5.00 or 7.00, but not 4.00 or 6.00.

JPQL also includes the following conditionals:

•  [NOT] BETWEEN: Shorthand for expressing that a value falls between two other values. The following two statements are synonymous:

  SELECT x FROM Magazine x WHERE x.price >= 3.00 AND x.price <= 5.00
  

  SELECT x FROM Magazine x WHERE x.price BETWEEN 3.00 AND 5.00
  

•  [NOT] LIKE: Performs a string comparison with wildcard support. The special character '_' in the parameter means to match any single character, and the special character '%' means to match any sequence of characters. The following statement matches title fields "JDJ" and "JavaPro", but not "IT Insider":

  SELECT x FROM Magazine x WHERE x.title LIKE 'J%'
  

  The following statement matches the title field "JDJ" but not "JavaPro":

  SELECT x FROM Magazine x WHERE x.title LIKE 'J__'
  

•  [NOT] IN: Specifies that the member must be equal to one element of the provided list. The following two statements are synonymous:

  SELECT x FROM Magazine x WHERE x.title IN ('JDJ', 'JavaPro', 'IT Insider')
  

  SELECT x FROM Magazine x WHERE x.title = 'JDJ' OR x.title = 'JavaPro' OR x.title = 'IT Insider'
  

•  IS [NOT] EMPTY: Specifies that the collection field holds no elements. For example:

  SELECT x FROM Magazine x WHERE x.articles is empty
  

  This statement will return all magazines whose articles member contains no elements.

•  IS [NOT] NULL: Specifies that the field is equal to null. For example:

  SELECT x FROM Magazine x WHERE x.publisher is null
  

  This statement will return all Magazine instances whose "publisher" field is set to null.

•  NOT: Negates the contained expression. For example, the following two statements are synonymous:

  SELECT x FROM Magazine x WHERE NOT(x.price = 10.0)
  

  SELECT x FROM Magazine x WHERE x.price <> 10.0
  

1.2.  Relation Traversal

Relations between objects can be traversed using Java-like syntax. For example, if the Magazine class has a field named "publisher" or type Company, that relation can be queried as follows:

SELECT x FROM Magazine x WHERE x.publisher.name = 'Random House'

This query returns all Magazine instances whose publisher field is set to a Company instance whose name is "Random House".

Single-valued relation traversal implies that the relation is not null. In SQL terms, this is known as an inner join. If you want to also include relations that are null, you can specify:

SELECT x FROM Magazine x WHERE x.publisher.name = 'Random House' or x.publisher is null

You can also traverse collection fields in queries, but you must declare each traversal in the from clause. Consider:

SELECT x FROM Magazine x, IN(x.articles) y WHERE y.authorName = 'John Doe'

This query says that for each Magazine x , traverse the articles relation and check each Articley, and pass the filter if y's authorName field is equal to "John Doe". In short, this query will return all magazines that have any articles written by John Doe.

Note

The IN() syntax can also be expressed with the keywords inner join. The statements SELECT x FROM Magazine x, IN(x.articles) y WHERE y.authorName = 'John Doe' and SELECT x FROM Magazine x inner join x.articles y WHERE y.authorName = 'John Doe'are synonymous.

1.3.  Fetch Joins

JPQL queries may specify one or more join fetch declarations, which allow the query to specify which fields in the returned instances will be pre-fetched.

SELECT x FROM Magazine x join fetch x.articles WHERE x.title = 'JDJ'

The query above returns Magazine instances and guarantees that the articles field will already be fetched in the returned instances.

Multiple fields may be specified in separate join fetch declarations:

SELECT x FROM Magazine x join fetch x.articles join fetch x.authors WHERE x.title = 'JDJ'

Note

Specifying the join fetch declaration is functionally equivalent to adding the fields to the Query's FetchConfiguration. See Section 6, “ Fetch Groups ”.

1.4.  JPQL Functions

As well as supporting direct field and relation comparisons, JPQL supports a pre-defined set of functions that you can apply.

•  CONCAT(string1, string2): Concatenates two string fields or literals. For example:

  SELECT x FROM Magazine x WHERE CONCAT(x.title, 's') = 'JDJs'
  

•  SUBSTRING(string, startIndex, length): Returns the part of the string argument starting at startIndex (1-based) and ending at length characters past startIndex.

  SELECT x FROM Magazine x WHERE SUBSTRING(x.title, 1, 1) = 'J'
  

•  TRIM([LEADING | TRAILING | BOTH] [character FROM] string: Trims the specified character from either the beginning ( LEADING ) end ( TRAILING) or both ( BOTH ) of the string argument. If no trim character is specified, the space character will be trimmed.

  SELECT x FROM Magazine x WHERE TRIM(BOTH 'J' FROM x.title) = 'D'
  

•  LOWER(string): Returns the lower-case of the specified string argument.

  SELECT x FROM Magazine x WHERE LOWER(x.title) = 'jdj'
  

•  UPPER(string): Returns the upper-case of the specified string argument.

  SELECT x FROM Magazine x WHERE UPPER(x.title) = 'JAVAPRO'
  

•  LENGTH(string): Returns the number of characters in the specified string argument.

  SELECT x FROM Magazine x WHERE LENGTH(x.title) = 3
  

•  LOCATE(searchString, candidateString [, startIndex]): Returns the first index of searchString in candidateString. Positions are 1-based. If the string is not found, returns 0.

  SELECT x FROM Magazine x WHERE LOCATE('D', x.title) = 2
  

•  ABS(number): Returns the absolute value of the argument.

  SELECT x FROM Magazine x WHERE ABS(x.price) >= 5.00
  

•  SQRT(number): Returns the square root of the argument.

  SELECT x FROM Magazine x WHERE SQRT(x.price) >= 1.00
  

•  MOD(number, divisor): Returns the modulo of number and divisor.

  SELECT x FROM Magazine x WHERE MOD(x.price, 10) = 0
  

•  CURRENT_DATE: Returns the current date.

•  CURRENT_TIME: Returns the current time.

•  CURRENT_TIMESTAMP: Returns the current timestamp.

1.5.  Polymorphic Queries

All JPQL queries are polymorphic, which means the from clause of a query includes not only instances of the specific entity class to which it refers, but all subclasses of that class as well. The instances returned by a query include instances of the subclasses that satisfy the query conditions. For example, the following query may return instances of Magazine , as well as Tabloid and Digest instances, where Tabloid and Digest are Magazine subclasses.

SELECT x FROM Magazine x WHERE x.price < 5

1.6.  Query Parameters

JPQL provides support for parameterized queries. Either named parameters or positional parameters may be specified in the query string. Parameters allow you to re-use query templates where only the input parameters vary. A single query can declare either named parameters or positional parameters, but is not allowed to declare both named and positional parameters.

public Query setParameter (int pos, Object value);

Specify positional parameters in your JPQL string using an integer prefixed by a question mark. You can then populate the Query object with positional parameter values via calls to the setParameter method above. The method returns theQuery instance for optional method chaining.

EntityManager em = ...
Query q = em.createQuery("SELECT x FROM Magazine x WHERE x.title = ?1 and x.price > ?2");
q.setParameter(1, "JDJ").setParameter(2, 5.0);
List<Magazine> results = (List<Magazine>) q.getResultList();

This code will substitute JDJ for the ?1 parameter and 5.0 for the ?2 parameter, then execute the query with those values.

public Query setParameter(String name, Object value);

Named parameter are denoted by prefixing an arbitrary name with a colon in your JPQL string. You can then populate the Query object with parameter values using the method above. Like the positional parameter method, this method returns the Query instance for optional method chaining.

EntityManager em = ...
Query q = em.createQuery("SELECT x FROM Magazine x WHERE x.title = :titleParam and x.price > :priceParam");
q.setParameter("titleParam", "JDJ").setParameter("priceParam", 5.0);
List<Magazine> results = (List<Magazine>) q.getResultList();

This code substitutes JDJ for the :titleParam parameter and 5.0 for the :priceParam parameter, then executes the query with those values.

1.7.  Query Hints

JPQL provides support for hints which are name/value pairs used to control locking and optimization keywords in sql. The following example shows how to use the JPA hint api to set the ReadLockMode and ResultCount in the OpenJPA fetch plan. This will result in the sql keywords OPTIMIZE FOR 2 ROWS and UPDATE to be emitted into the sql provided that a pessimistic LockManager is being used.

Example 10.1.  Query Hints

...
Query q = em.createQuery("select m from Magazine m where ... ");
q.setHint("openjpa.hint.OptimizeResultCount", new Integer(2));
q.setHint("openjpa.FetchPlan.ReadLockMode","WRITE");
List r = q.getResultList();
...

Invalid hints or hints which can not be processed by a particular database are ignored. Otherwise, invalid hints will result in an ArgumentException being thrown.

1.7.1.  Locking Hints

To avoid deadlock and optimistic update exceptions among multiple updaters, use a pessimistic LockManager, specified in the persistence unit definition, and use a hint name of "openjpa.FetchPlan.ReadLockMode" on queries for entities that must be locked for serialization. The value of ReadLockMode can be either "READ" or "WRITE". This results in FOR UPDATE or USE AND KEEP UPDATE LOCKS in sql.

Using a ReadLockMode hint with JPA optimistic locking ( i.e. specifying LockManager = "version") will result in the entity version field either being reread at end of transaction in the case of a value of "READ" or the version field updated at end of transaction in the case of "WRITE". You must define a version field in the entity mapping when using a version LockManager and using ReadLockMode.

Table 10.1.  Interaction of ReadLockMode hint and LockManager

ReadLockMode	LockManager=pessimistic	LockManager=version
READ	sql with UPDATE	sql without update; reread version field at the end of transaction and check for no change.
WRITE	sql with UPDATE	sql without update; force update version field at the end of transaction
not specified	sql without update	sql without update

1.7.2.  Result Set Size Hint

To specify a result set size hint to those databases that support it, specify a hint name of "openjpa.hint.OptimizeResultCount" with an integer value greater than zero. This causes the sql keyword OPTIMIZE FOR to be generated.

1.7.3.  Isolation Level Hint

To specify an isolation level, specify a hint name of "openjpa.FetchPlan.Isolation". The value will be used to specify isolation level using the sql WITH <isolation> clause for those databases that support it. This hint only works in conjunction with the ReadLockMode hint.

1.7.4.  Other Fetchplan Hints

Any property of an OpenJPA FetchPlan can be changed using a hint by using a name of the form "openjpa.FetchPlan."<property name>.Valid property names include : MaxFetchDepth, FetchBatchSize, LockTimeOut, EagerFetchMode, SubclassFetchMode andIsolation.

1.7.5.  Oracle Query Hints

The hint name "openjpa.hint.OracleSelectHint" can be used to specify a string value of an Oracle query hint that will inserted into sql for an Oracle database.See Section 15.1, “ Using Query Hints with Oracle ” for an example.

1.7.6.  Named Query Hints

Hints can also be included as part of a NamedQuery definition.

Example 10.2.  Named Query using Hints

...
@NamedQuery(name=" magsOverPrice",
query="SELECT x FROM Magazine x WHERE x.price > ?1",
hints={ @QueryHint  (name="openjpa.hint.OptimizeResultCount", value="2"),
        @QueryHint (name="openjpa.FetchPlan.ReadLockMode",value="WRITE")} )
...

1.8.  Ordering

JPQL queries may optionally contain an order by clause which specifies one or more fields to order by when returning query results. You may follow the order by field clause with the asc or desc keywords, which indicate that ordering should be ascending or descending, respectively. If the direction is omitted, ordering is ascending by default.

SELECT x FROM Magazine x order by x.title asc, x.price desc

The query above returns Magazine instances sorted by their title in ascending order. In cases where the titles of two or more magazines are the same, those instances will be sorted by price in descending order.

1.9.  Aggregates

JPQL queries can select aggregate data as well as objects. JPQL includes the min, max, avg, and count aggregates. These functions can be used for reporting and summary queries.

The following query will return the average of all the prices of all the magazines:

EntityManager em = ...
Query q = em.createQuery("SELECT AVG(x.price) FROM Magazine x");
Number result = (Number) q.getSingleResult();

The following query will return the highest price of all the magazines titled "JDJ":

EntityManager em = ...
Query q = em.createQuery("SELECT MAX(x.price) FROM Magazine x WHERE x.title = 'JDJ'");
Number result = (Number) q.getSingleResult();

1.10.  Named Queries

Query templates can be statically declared using the NamedQuery and NamedQueries annotations. For example:

@Entity
@NamedQueries({
    @NamedQuery(name="magsOverPrice",
        query="SELECT x FROM Magazine x WHERE x.price > ?1"),
    @NamedQuery(name="magsByTitle",
        query="SELECT x FROM Magazine x WHERE x.title = :titleParam")
})
public class Magazine {
    ...
}

These declarations will define two named queries called magsOverPrice and magsByTitle.

public Query createNamedQuery(String name);

You retrieve named queries with the above EntityManager method. For example:

EntityManager em = ...
Query q = em.createNamedQuery("magsOverPrice");
q.setParameter(1, 5.0f);
List<Magazine> results = (List<Magazine>) q.getResultList();

EntityManager em = ...
Query q = em.createNamedQuery("magsByTitle");
q.setParameter("titleParam", "JDJ");
List<Magazine> results = (List<Magazine>) q.getResultList();

1.11.  Delete By Query

Queries are useful not only for finding objects, but for efficiently deleting them as well. For example, you might delete all records created before a certain date. Rather than bring these objects into memory and delete them individually, JPA allows you to perform a single bulk delete based on JPQL criteria.

Delete by query uses the same JPQL syntax as normal queries, with one exception: begin your query string with the delete keyword instead of the select keyword. To then execute the delete, you call the following Query method:

public int executeUpdate();

This method returns the number of objects deleted. The following example deletes all subscriptions whose expiration date has passed.

Example 10.3.  Delete by Query

Query q = em.createQuery("DELETE FROM Subscription s WHERE s.subscriptionDate < :today");
q.setParameter("today", new Date());
int deleted = q.executeUpdate();

1.12.  Update By Query

Similar to bulk deletes, it is sometimes necessary to perform updates against a large number of queries in a single operation, without having to bring all the instances down to the client. Rather than bring these objects into memory and modifying them individually, JPA allows you to perform a single bulk update based on JPQL criteria.

Update by query uses the same JPQL syntax as normal queries, except that the query string begins with the update keyword instead of select. To execute the update, you call the following Query method:

public int executeUpdate();

This method returns the number of objects updated. The following example updates all subscriptions whose expiration date has passed to have the "paid" field set to true..

Example 10.4.  Update by Query

Query q = em.createQuery("UPDATE Subscription s SET s.paid = :paid WHERE s.subscriptionDate < :today");
q.setParameter("today", new Date());
q.setParameter("paid", true);
int updated = q.executeUpdate();

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