idgeneratorbean.java

J2EE开发与Weblogic一书中的源代码

package webauction.ejb;

import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.ResultSet;
import java.sql.Statement;
import java.sql.SQLException;

import javax.ejb.EJBException;
import javax.ejb.SessionBean;
import javax.ejb.SessionContext;
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.sql.DataSource;

public class IDGeneratorBean implements SessionBean {
    private static final boolean SUPPORTS_SEQUENCES = false;
    private SessionContext ctx;
    private DataSource dataSource;
    private String jdbcStatement = null;
    private String sequenceName;

    public void setSessionContext(SessionContext c) {
        ctx = c;
        Context ic;
        try {
            ic = new InitialContext();
            String poolName = (String) ic.lookup(EjbConstants.LOOKUP_POOL_NAME);
            dataSource =
                (DataSource) ic.lookup("java:/comp/env/" + poolName);
            sequenceName = (String) ic.lookup(EjbConstants.LOOKUP_SEQ_NAME);
        } catch (NamingException ne) {
            ne.printStackTrace();
            throw new EJBException("Unable to get datasource: " + ne);
        }
    }

    private void setJdbcStatement() {
        // Several databases do not support sequences. We simulate them with
        // an update followed by a select.

        // Oracle supports sequences but uses a slightly different set of
        // SQL.  We check for oracle and change the JDBC statement.
        try {
            DatabaseMetaData md = dataSource.getConnection().getMetaData();
            String driverName = md.getDriverName();
            if (SUPPORTS_SEQUENCES) {
                if (driverName
                    .startsWith("Weblogic, Inc. Java-OCI JDBC Driver")
                    || driverName.startsWith("Oracle JDBC driver")) {
                    // Oracle syntax
                    jdbcStatement =
                        "select " + sequenceName + ".nextval FROM dual";
                } else {
                    // SQL syntax
                    jdbcStatement = "select nextval('" + sequenceName + "')";
                }
            }
        } catch (SQLException sqe) {
            throw new EJBException(sqe);
        }
    }
    
    private int getNextValueFromSequence(Connection conn) throws SQLException {
        Statement st = conn.createStatement();
        ResultSet rs = st.executeQuery(jdbcStatement);
        if (rs.next()) {
            return rs.getInt(1);
        } else {
            throw new EJBException(
                "Query '" + jdbcStatement + "' did not return any results");
        }
    }

    private int getNextValueFromTable(Connection conn) throws SQLException {
        Statement st = conn.createStatement();
        st.executeUpdate("update " + sequenceName + " set seqNo = seqNo + 1");
        st = conn.createStatement();
        ResultSet rs = st.executeQuery("select seqNo from " + sequenceName);
        if (rs.next()) {
            return rs.getInt(1);
        } else {
            throw new EJBException(
                "Query '" + jdbcStatement + "' did not return any results");
        }
    }

    public int getNextValue() {
        try {
        	if(null == jdbcStatement) {
        		setJdbcStatement();
        	}
            Connection conn = dataSource.getConnection();
            if (SUPPORTS_SEQUENCES) {
                return getNextValueFromSequence(conn);
            } else {
                return getNextValueFromTable(conn);
            }
        } catch (SQLException sqe) {
            throw new EJBException(sqe);
        }
    }

    public void ejbCreate() {}
    public void ejbRemove() {}
    public void ejbPassivate() {}
    public void ejbActivate() {}
}