Java Issuing Queries and Retrieving Results

2.5.8 Java

The JDBC int erface provides specific obj ect t ypes for t he various phases of query processing. Queries are issued in JDBC by passing SQL st rings t o Java obj ect s of one t ype. The result s, if t here are any, are ret urned as obj ect s of anot her t ype. Problem s t hat occur while accessing t he dat abase cause except ions t o be t hrown. To issue a query, t he first st ep is t o get a Statement obj ect by calling t he createStatement m et hod of your Connection obj ect : Statement s = conn.createStatement ; Then use t he Statement obj ect t o send t he query t o t he server. JDBC provides several m et hods for doing t his. Choose t he one t hat s appropriat e for t he t ype of st at em ent you want t o issue: executeUpdate for st at em ent s t hat dont ret urn a result set , executeQuery for st at em ent s t hat do, and execute when you dont know. The executeUpdate m et hod sends a query t hat generat es no result set t o t he server and ret urns a count indicat ing t he num ber of rows t hat were affect ed. When youre done wit h t he st at em ent obj ect , close it . The following exam ple illust rat es t his sequence of event s: try { Statement s = conn.createStatement ; int count = s.executeUpdate DELETE FROM profile WHERE cats = 0; s.close ; close statement System.out.println count + rows were deleted; } catch Exception e { Cookbook.printErrorMessage e; } For st at em ent s t hat ret urn a result set , use executeQuery . Then get a result set obj ect and use it t o ret rieve t he row values. When youre done, close bot h t he result set and st at em ent obj ect s: try { Statement s = conn.createStatement ; s.executeQuery SELECT id, name, cats FROM profile; ResultSet rs = s.getResultSet ; int count = 0; while rs.next loop through rows of result set { int id = rs.getInt 1; extract columns 1, 2, and 3 String name = rs.getString 2; int cats = rs.getInt 3; System.out.println id: + id + , name: + name + , cats: + cats; ++count; } rs.close ; close result set s.close ; close statement System.out.println count + rows were returned; } catch Exception e { Cookbook.printErrorMessage e; } The ResultSet obj ect ret urned by t he getResultSet m et hod of your Statement obj ect has a num ber of m et hods of it s own, such as next t o fet ch row s and various get XXX m et hods t hat access colum ns of t he current row. I nit ially t he result set is posit ioned j ust before t he first row of t he set . Call next t o fet ch each row in succession unt il it ret urns false, indicat ing t hat t here are no m ore rows. To det erm ine t he num ber of rows in a result set , count t hem yourself, as shown in t he preceding exam ple. Colum n values are accessed using m et hods such as getInt , getString , getFloat , and getDate . To obt ain t he colum n value as a generic obj ect , use getObject . The get XXX calls can be invoked wit h an argum ent indicat ing eit her colum n posit ion beginning at 1, not 0 or colum n nam e. The previous exam ple shows how t o ret rieve t he id , name , and cats colum ns by posit ion. To access colum ns by nam e inst ead, t he row- fet ching loop of t hat exam ple can be rewrit t en as follows: while rs.next loop through rows of result set { int id = rs.getInt id; String name = rs.getString name; int cats = rs.getInt cats; System.out.println id: + id + , name: + name + , cats: + cats; ++count; } You can ret rieve a given colum n value using any get XXX call t hat m akes sense for t he colum n t ype. For exam ple, you can use getString t o ret rieve any colum n value as a st r ing: String id = rs.getString id; String name = rs.getString name; String cats = rs.getString cats; System.out.println id: + id + , name: + name + , cats: + cats; Or you can use getObject t o ret rieve values as generic obj ect s and convert t he values as necessary. The following code uses toString t o convert obj ect values t o print able form : Object id = rs.getObject id; Object name = rs.getObject name; Object cats = rs.getObject cats; System.out.println id: + id.toString + , name: + name.toString + , cats: + cats.toString ; To find out how m any colum ns are in each row, access t he result set s m et adat a. The following code uses t he colum n count t o print each rows colum ns as a com m a- separat ed list of values: try { Statement s = conn.createStatement ; s.executeQuery SELECT FROM profile; ResultSet rs = s.getResultSet ; ResultSetMetaData md = rs.getMetaData ; get result set metadata int ncols = md.getColumnCount ; get column count from metadata int count = 0; while rs.next loop through rows of result set { for int i = 0; i ncols; i++ loop through columns { String val = rs.getString i+1; if i 0 System.out.print , ; System.out.print val; } System.out.println ; ++count; } rs.close ; close result set s.close ; close statement System.out.println count + rows were returned; } catch Exception e { Cookbook.printErrorMessage e; } The t hird JDBC query- execut ing m et hod, execute , works for eit her t ype of query. I t s part icularly useful when you receive a query st ring from an ext ernal source and dont know whet her or not it generat es a result set . The ret urn value from execute indicat es t he query t ype so t hat you can process it appropriat ely: if execute ret urns t rue, t here is a result set , ot herwise not . Typically youd use it som et hing like t his, where queryStr represent s an arbit rary SQL st at em ent : try { Statement s = conn.createStatement ; if s.execute queryStr { there is a result set ResultSet rs = s.getResultSet ; ... process result set here ... rs.close ; close result set } else { there is no result set, just print the row count System.out.println s.getUpdateCount + rows were affected; } s.close ; close statement } catch Exception e { Cookbook.printErrorMessage e; } Closing JDBC Statement and Result Set Objects The JDBC query- issuing exam ples in t his sect ion close t he st at em ent and result set obj ect s explicit ly when t hey are done wit h t hose obj ect s. Som e Java im plem ent at ions close t hem aut om at ically when you close t he connect ion. However, buggy im plem ent at ions m ay fail t o do t his properly, so it s best not t o rely on t hat behavior. Close t he obj ect s yourself when youre done wit h t hem t o avoid difficult ies.

2.6 Moving Around Within a Result Set