The abstract mapper can also help find employees in Java

Draw QR Code ISO/IEC18004 in Java The abstract mapper can also help find employees
The abstract mapper can also help find employees
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class EmployeeMapper public List findAll() { return findAll(findAllStatement); } private static final String findAllStatement = "SELECT " + COLUMN_LIST + " FROM employees employee" + " ORDER BY employeelastname"; class AbstractMapper protected List findAll(String sql) { PreparedStatement stmt = null; ResultSet rs = null; try { List result = new ArrayList(); stmt = DBprepare(sql); rs = stmtexecuteQuery(); while (rsnext()) resultadd(load(rs)); return result; } catch (SQLException e) { throw new ApplicationException(e); } finally {DBcleanUp(stmt, rs); } }
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All of this works quite well and is pretty simple to follow Still, there's a problem in the number of queries, and that is that each employee takes two SQL queries to load Although we can load the basic employee data for many employees in a single query, we still need one query per employee to load the skills Thus, loading a hundred employees takes 101 queries
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Example: Using a Single Query for Multiple Employees (Java)
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It's possible to bring back many employees, with their skills, in a single query This is a good example of
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multitable query optimization, which is certainly more awkward For that reason do this when you need to, rather than every time It's better to put more energy into speeding up your slow queries than into many queries that are less important
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The first case we'll look at is a simple one where we pull back all the skills for an employee in the same query that holds the basic data To do this I'll use a more complex SQL statement that joins across all three tables
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class EmployeeMapper protected String findStatement() { return "SELECT " + COLUMN_LIST + " FROM employees employee, skills skill, employeeSkills es" + " WHERE employeeID = esemployeeID AND skillID = esskillID AND employeeID = "; } public static final String COLUMN_LIST = " employeeID, employeelastname, employeefirstname, " + " esskillID, esemployeeID, skillID skillID, " + SkillMapperCOLUMN_LIST;
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The abstractFind and load methods on the superclass are the same as in the previous example, so I won't repeat them here The employee mapper loads its data differently to take advantage of the multiple data rows
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class EmployeeMapper protected DomainObject doLoad(Long id, ResultSet rs) throws SQLException { Employee result = (Employee) loadRow(id, rs); loadSkillData(result, rs); while (rsnext()){ AssertisTrue(rowIsForSameEmployee(id, rs)); loadSkillData(result, rs); } return result; } protected DomainObject loadRow(Long id, ResultSet rs) throws SQLException { Employee result = new Employee(id); resultsetFirstName(rsgetString("firstname")); resultsetLastName(rsgetString("lastname")); return result; } private boolean rowIsForSameEmployee(Long id, ResultSet rs) throws SQLException { return idequals(new Long(rsgetLong(1))); } private void loadSkillData(Employee person, ResultSet rs) throws SQLException { Long skillID = new Long(rsgetLong("skillID")); personaddSkill ((Skill)MapperRegistryskill()loadRow(skillID, rs)); }
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In this case the load method for the employee mapper actually runs through the rest of the result set to load in all the data
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All is simple when we're loading the data for a single employee However, the real benefit of this multitable query appears when we want to load lots of employees Getting the reading right can be tricky, particularly when we don't want to force the result set to be grouped by employees At this point it's handy to introduce a helper class to go through the result set by focusing on the associative table itself, loading up the employees and skills as it goes along
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I'll begin with the SQL and the call to the special loader class
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class EmployeeMapper public List findAll() { return findAll(findAllStatement); } private static final String findAllStatement = "SELECT " + COLUMN_LIST + " FROM employees employee, skills skill, employeeSkills es" + " WHERE employeeID = esemployeeID AND skillID = esskillID" + " ORDER BY employeelastname"; protected List findAll(String sql) { AssociationTableLoader loader = new AssociationTableLoader(this, new SkillAdder()); return loaderrun(findAllStatement); } class AssociationTableLoader private AbstractMapper sourceMapper; private Adder targetAdder; public AssociationTableLoader(AbstractMapper primaryMapper, Adder targetAdder) { thissourceMapper = primaryMapper; thistargetAdder = targetAdder; }
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Don't worry about the skillAdder that will become a bit clearer later For the moment notice that we construct the loader with a reference to the mapper and then tell it to perform a load with a suitable query This is the typical structure of a method object A method object [Beck Patterns] is a way of turning a complicated method into an object on its own The great advantage of this is that it allows you to put values in fields instead of passing them around in parameters The usual way of using a method object is to create it, fire it up, and then let it die once its duty is done
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