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JDK 5 Documentation v1.1.8, Java 2 SDK 英文文档

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5 - ResultSet

This overview is excerpted from JDBCTM Database Access from JavaTM: A Tutorial and Annotated Reference, currently in progress at JavaSoft. This book, both a tutorial and the definitive reference manual for JDBC, will be published in the spring of 1997 by Addison-Wesley Publishing Company as part of the Java series.

5.1    Overview

A ResultSet contains all of the rows which satisfied the conditions in an SQL statement, and it provides access to the data in those rows through a set of get methods that allow access to the various columns of the current row. The ResultSet.next method is used to move to the next row of the ResultSet, making the next row become the current row.

The general form of a result set is a table with column headings and the corresponding values returned by a query. For example, if your query is SELECT a, b, c FROM Table1, your result set will have the following form: 

        a          b            c
        --------   ---------    -------- 
        12345      Cupertino    CA
        83472      Redmond      WA
        83492      Boston       MA

The following code fragment is an example of executing an SQL statement that will return a collection of rows, with column 1 as an int, column 2 as a String, and column 3 as an array of bytes: 

    
    java.sql.Statement stmt = conn.createStatement();
    ResultSet r = stmt.executeQuery("SELECT a, b, c FROM Table1");
    while (r.next()) 
    {
      // print the values for the current row.
      int i = r.getInt("a");
      String s = r.getString("b");
      float f = r.getFloat("c");
      System.out.println("ROW = " + i + " " + s + " " + f);
    }

5.1.1     Rows and Cursors

A ResultSet maintains a cursor which points to its current row of data. The cursor moves down one row each time the method next is called. Initially it is positioned before the first row, so that the first call to next puts the cursor on the first row, making it the current row. ResultSet rows are retrieved in sequence from the top row down as the cursor moves down one row with each successive call to next.

A cursor remains valid until the ResultSet object or its parent Statement object is closed.

In SQL, the cursor for a result table is named. If a database allows positioned updates or positioned deletes, the name of the cursor needs to be supplied as a parameter to the update or delete command. This cursor name can be obtained by calling the method getCursorName.

Note that not all DBMSs support positioned update and delete. The DatabaseMetaData.supportsPositionedDelete and supportsPositionedUpdate methods can be used to discover whether a particular connection supports these operations. When they are supported, the DBMS/driver must ensure that rows selected are properly locked so that positioned updates do not result in update anomalies or other concurrency problems.

5.1.2     Columns

The getXXX methods provide the means for retrieving column values from the current row. Within each row, column values may be retrieved in any order, but for maximum portability, one should retrieve values from left to right and read column values only once.

Either the column name or the column number can be used to designate the column from which to retrieve data. For example, if the second column of a ResultSet object rs is named "title" and stores values as strings, either of the following will retrieve the value stored in that column: 

    String s = rs.getString("title");
    String s = rs.getString(2);
Note that columns are numbered from left to right starting with column 1. Also, column names used as input to getXXX methods are case insensitive.

The option of using the column name was provided so that a user who specifies column names in a query can use those same names as the arguments to getXXX methods. If, on the other hand, the select statement does not specify column names (as in "select * from table1" or in cases where a column is derived), column numbers should be used. In such situations, there is no way for the user to know for sure what the column names are.

In some cases, it is possible for a SQL query to return a result set that has more than one column with the same name. If a column name is used as the parameter to a getXXX method, getXXX will return the value of the first matching column name. Thus, if there are multiple columns with the same name, one needs to use a column index to be sure that the correct column value is retrieved. It may also be slightly more efficient to use column numbers.

Information about the columns in a ResultSet is available by calling the method ResultSet.getMetaData. The ResultSetMetaData object returned gives the number, types, and properties of its ResultSet object's columns.

If the name of a column is known, but not its index, the method findColumn can be used to find the column number.

5.1.3     Data Types and Conversions

For the getXXX methods, the JDBC driver attempts to convert the underlying data to the specified Java type and then returns a suitable Java value. For example, if the getXXX method is getString, and the data type of the data in the underlying database is VARCHAR, the JDBC driver will convert VARCHAR to Java String. The return value of getString will be a Java String object.

The following table shows which JDBC types a getXXX method is allowed to retrieve and which JDBC types (generic SQL types) are recommended for it to retrieve. A small x indicates a legal getXXX method for a particular data type; a large X indicates the recommended getXXX method for a data type. For example, any getXXX method except getBytes or getBinaryStream can be used to retrieve the value of a LONGVARCHAR, but getAsciiStream or getUnicodeStream are recommended, depending on which data type is being returned. The method getObject will return any data type as a Java Object and is useful when the underlying data type is a database-specific abstract type or when a generic application needs to be able to accept any data type.

Use of ResultSet.getXXX methods to retrieve common JDBC data types.

An "x" indicates that the getXXX method may legally be used to retrieve the given JDBC type.

An "X" indicates that the getXXX method is recommended for retrieving the given JDBC type.

  T
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T 		S
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getByte X x x x x x x x x x x x x            
getShort x X x x x x x x x x x x x            
getInt x x X x x x x x x x x x x            
getLong x x x X x x x x x x x x x            
getFloat x x x x X x x x x x x x x            
getDouble x x x x x X X x x x x x x            
getBigDecimal x x x x x x x X X x x x x            
getBoolean x x x x x x x x x X x x x            
getString x x x x x x x x x x X X x x x x x x x
getBytes                           X X x      
getDate                     x x x       X   x
getTime                     x x x         X x
getTimestamp                     x x x       x   X
getAsciiStream                     x x X x x x      
getUnicodeStream                     x x X x x x      
getBinaryStream                           x x X      
getObject x x x x x x x x x x x x x x x x x x x

5.1.4     Using Streams for Very Large Row Values

ResultSet makes it possible to retrieve arbitrarily large LONGVARBINARY or LONGVARCHAR data. The methods getBytes and getString return data as one large chunk (up to the limits imposed by the return value of Statement.getMaxFieldSize). However, it may be more convenient to retrieve very large data in smaller, fixed-size chunks. This is done by having the ResultSet class return java.io.Input streams from which data can be read in chunks. Note that these streams must be accessed immediately because they will be closed automatically on the next getXXX call on ResultSet. (This behavior is imposed by underlying implementation constraints on large blob access.)

The JDBC API has three separate methods for getting streams, each with a different return value:

Note that this differs from Java streams, which return untyped bytes and can (for example) be used for both ASCII and Unicode characters.

The following code gives an example of using getAsciiStream: 

    java.sql.Statement stmt = con.createStatement();
    ResultSet r = stmt.executeQuery("SELECT x FROM Table2");
    // Now retrieve the column 1 results in 4 K chunks:
    byte buff = new byte[4096];
    while (r.next()) { 
      Java.io.InputStream fin = r.getAsciiStream(1);
      for (;;) {
        int size = fin.read(buff);
        if (size == -1) { // at end of stream
              break;
        }
        // Send the newly-filled buffer to some ASCII output stream:
        output.write(buff, 0, size);
      }
    }

5.1.5     NULL Result Values

To determine if a given result value is JDBC NULL, one must first read the column and then use the ResultSet.wasNull method to discover if the read returned a JDBC NULL.

When one has read a JDBC NULL using one of the ResultSet.getXXX methods, the method wasNull will return one of the following:

5.1.6     Optional or Multiple Result Sets

Normally SQL statements are executed using either executeQuery (which returns a single ResultSet) or executeUpdate (which can be used for any kind of database modification statement and which returns a count of the rows updated). However, under some circumstances an application may not know whether a given statement will return a result set until the statement has executed. In addition, some stored procedures may return several different result sets and/or update counts.

To accommodate these situations, JDBC provides a mechanism so that an application can execute a statement and then process an arbitrary collection of result sets and update counts. This mechanism is based on first calling a fully general execute method, and then calling three other methods, getResultSet, getUpdateCount, and getMoreResults. These methods allow an application to explore the statement results one at a time and to determine if a given result was a ResultSet or an update count.

You do not need to do anything to close a ResultSet; it is automatically closed by the Statement that generated it when that Statement is closed, is re-executed, or is used to retrieve the next result from a sequence of multiple results.


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