04-dbutils源码之各类ResultSetHandler实现类

时间 2019-11-09

标签 dbutils 源码各类 resultsethandler 实现繁體版

原文原文链接

今天主要讲解下经常使用的Handler，Handler讲完以后，就能对dbutils总体有了了解了。 java

来看下ResultSetHandler接口

/**
 * Implementations of this interface convert ResultSets into other objects.
 * 此接口的实现类将结果集转化为其余的对象
 * @param <T> the target type the input ResultSet will be converted to.
 */
public interface ResultSetHandler<T> {

    /**
     * Turn the <code>ResultSet</code> into an Object.
     * 将结果集转换成对象
     * @param rs The <code>ResultSet</code> to handle.  It has not been touched
     * before being passed to this method.
     *
     * @return An Object initialized with <code>ResultSet</code> data. It is
     * legal for implementations to return <code>null</code> if the
     * <code>ResultSet</code> contained 0 rows.
     *
     * @throws SQLException if a database access error occurs
     */
    T handle(ResultSet rs) throws SQLException;

}

它就只有一个方法声明，很简单，就不讲什么。 sql

在这里我会讲解：BeanHandler、ArrayHandler、BeanListHandler、MapHandler、ScalarHandler这5个Handler。剩下的AbstractKeyedHandler以及实现类和AbstractListHandler及实现类，会在下一篇里讲到。数据库

BeanHandler源码：

/**
 * <code>ResultSetHandler</code> implementation that converts the first
 * <code>ResultSet</code> row into a JavaBean. This class is thread safe.
 * ResultSetHandler实现类将结果集的第一行转换成一个javabean
 * @param <T> the target bean type
 * @see org.apache.commons.dbutils.ResultSetHandler
 */
public class BeanHandler<T> implements ResultSetHandler<T> {

    /**
     * The Class of beans produced by this handler.
     * 这个handler要返回的bean的Class
     */
    private final Class<T> type;

    /**
     * The RowProcessor implementation to use when converting rows
     * into beans.
     * 将行转换成bean时，使用RowProcessor的哪一个实现类。默认使用的是BasicRowProcessor
     */
    private final RowProcessor convert;

    /**
     * Creates a new instance of BeanHandler.
     * 建立一个BeanHandler对象，默认使用的是BasicRowProcessor
     * @param type The Class that objects returned from <code>handle()</code>
     * are created from.
     */
    public BeanHandler(Class<T> type) {
    	//static final RowProcessor ROW_PROCESSOR = new BasicRowProcessor();
        this(type, ArrayHandler.ROW_PROCESSOR);
    }

    /**
     * Creates a new instance of BeanHandler.
     * 建立一个BeanHandler对象。能够自定义RowProcessor
     * @param type The Class that objects returned from <code>handle()</code>
     * are created from.
     * @param convert The <code>RowProcessor</code> implementation
     * to use when converting rows into beans.
     */
    public BeanHandler(Class<T> type, RowProcessor convert) {
        this.type = type;
        this.convert = convert;
    }

    /**
     * Convert the first row of the <code>ResultSet</code> into a bean with the
     * <code>Class</code> given in the constructor.
     * 将RS中的第一行记录转换成一个bean对象，这个bean对象经过在构造器中给定的Class建立。使用了反射。
     * @param rs <code>ResultSet</code> to process.
     * @return An initialized JavaBean or <code>null</code> if there were no
     * rows in the <code>ResultSet</code>.
     *
     * @throws SQLException if a database access error occurs
     * @see org.apache.commons.dbutils.ResultSetHandler#handle(java.sql.ResultSet)
     */
    @Override
    public T handle(ResultSet rs) throws SQLException {
    	/*
    	 * 若是有RS中有记录的时候才进行处理，不然返回null。属性convert在构造函数中赋值了
    	 */
        return rs.next() ? this.convert.toBean(rs, this.type) : null;
    }

}

BeanHandler的实现简单，就是调用了BasicRowProcessor中的toBean方法。 apache

BeanListHandler源码：

/**
 * <code>ResultSetHandler</code> implementation that converts a
 * <code>ResultSet</code> into a <code>List</code> of beans. This class is
 * thread safe.
 * 此实现类将结果集转换成bean的集合
 * @param <T> the target bean type
 * @see org.apache.commons.dbutils.ResultSetHandler
 */
public class BeanListHandler<T> implements ResultSetHandler<List<T>> {

    /**
     * The Class of beans produced by this handler.
     */
    private final Class<T> type;

    /**
     * The RowProcessor implementation to use when converting rows
     * into beans.
     * 使用的仍是BasicRowProcessor
     */
    private final RowProcessor convert;

    /**
     * Creates a new instance of BeanListHandler.
     *
     * @param type The Class that objects returned from <code>handle()</code>
     * are created from.
     */
    public BeanListHandler(Class<T> type) {
        this(type, ArrayHandler.ROW_PROCESSOR);
    }

    /**
     * Creates a new instance of BeanListHandler.
     *
     * @param type The Class that objects returned from <code>handle()</code>
     * are created from.
     * @param convert The <code>RowProcessor</code> implementation
     * to use when converting rows into beans.
     */
    public BeanListHandler(Class<T> type, RowProcessor convert) {
        this.type = type;
        this.convert = convert;
    }

    /**
     * Convert the whole <code>ResultSet</code> into a List of beans with
     * the <code>Class</code> given in the constructor.
     * 调用了BasicRowProcessor的toBeanList方法。
     * @param rs The <code>ResultSet</code> to handle.
     *
     * @return A List of beans, never <code>null</code>.
     *
     * @throws SQLException if a database access error occurs
     * @see org.apache.commons.dbutils.RowProcessor#toBeanList(ResultSet, Class)
     */
    @Override
    public List<T> handle(ResultSet rs) throws SQLException {
        return this.convert.toBeanList(rs, type);
    }
}

BeanListHandler跟BeanHandler实现相似。编程

ArrayHandler源码：

/**
 * <code>ResultSetHandler</code> implementation that converts a
 * <code>ResultSet</code> into an <code>Object[]</code>. This class is
 * thread safe.
 * <p>
 * 将ResultSet中的一行记录变成一个Object[]数组，此类是线程安全的
 * </p>
 * @see org.apache.commons.dbutils.ResultSetHandler
 */
public class ArrayHandler implements ResultSetHandler<Object[]> {

    /**
     * Singleton processor instance that handlers share to save memory.  Notice
     * the default scoping to allow only classes in this package to use this
     * instance.
     */
    static final RowProcessor ROW_PROCESSOR = new BasicRowProcessor();

    /**
     * The RowProcessor implementation to use when converting rows
     * into arrays.
     */
    private final RowProcessor convert;

    /**
     * Creates a new instance of ArrayHandler using a
     * <code>BasicRowProcessor</code> for conversion.
     */
    public ArrayHandler() {
        this(ROW_PROCESSOR);
    }

    /**
     * Creates a new instance of ArrayHandler.
     *
     * @param convert The <code>RowProcessor</code> implementation
     * to use when converting rows into arrays.
     */
    public ArrayHandler(RowProcessor convert) {
        super();
        this.convert = convert;
    }

    /**
     * Places the column values from the first row in an <code>Object[]</code>.
     * 将第一行的记录编程Object[]
     * @param rs <code>ResultSet</code> to process.
     * @return An Object[] or <code>null</code> if there are no rows in the
     * <code>ResultSet</code>.
     *
     * @throws SQLException if a database access error occurs
     * @see org.apache.commons.dbutils.ResultSetHandler#handle(java.sql.ResultSet)
     */
    @Override
    public Object[] handle(ResultSet rs) throws SQLException {
    	//有记录的话，就将第一行记录变成一个Object[]
        return rs.next() ? this.convert.toArray(rs) : null;
    }

}

MapHandler源码：

/**
 * <code>ResultSetHandler</code> implementation that converts the first
 * <code>ResultSet</code> row into a <code>Map</code>. This class is thread
 * safe.
 *
 * @see org.apache.commons.dbutils.ResultSetHandler
 */
public class MapHandler implements ResultSetHandler<Map<String, Object>> {

    /**
     * The RowProcessor implementation to use when converting rows
     * into Maps.
     */
    private final RowProcessor convert;

    /**
     * Creates a new instance of MapHandler using a
     * <code>BasicRowProcessor</code> for conversion.
     */
    public MapHandler() {
        this(ArrayHandler.ROW_PROCESSOR);
    }

    /**
     * Creates a new instance of MapHandler.
     *
     * @param convert The <code>RowProcessor</code> implementation
     * to use when converting rows into Maps.
     */
    public MapHandler(RowProcessor convert) {
        super();
        this.convert = convert;
    }

    /**
     * Converts the first row in the <code>ResultSet</code> into a
     * <code>Map</code>.
     * @param rs <code>ResultSet</code> to process.
     * @return A <code>Map</code> with the values from the first row or
     * <code>null</code> if there are no rows in the <code>ResultSet</code>.
     *
     * @throws SQLException if a database access error occurs
     *
     * @see org.apache.commons.dbutils.ResultSetHandler#handle(java.sql.ResultSet)
     */
    @Override
    public Map<String, Object> handle(ResultSet rs) throws SQLException {
        return rs.next() ? this.convert.toMap(rs) : null;
    }

}

ScalarHandler源码：

/**
 * <code>ResultSetHandler</code> implementation that converts one
 * <code>ResultSet</code> column into an Object. This class is thread safe.
 * 将结果集中第一行的第一列的值变成一个对象。便是单值的。
 * @param <T> The type of the scalar
 * @see org.apache.commons.dbutils.ResultSetHandler
 */
public class ScalarHandler<T> implements ResultSetHandler<T> {

    /**
     * The column number to retrieve.
     */
    private final int columnIndex;

    /**
     * The column name to retrieve.  Either columnName or columnIndex
     * will be used but never both.
     */
    private final String columnName;

    /**
     * Creates a new instance of ScalarHandler.  The first column will
     * be returned from <code>handle()</code>.
     * 默认会返回结果集中第一行的第一列的值
     */
    public ScalarHandler() {
        this(1, null);
    }

    /**
     * Creates a new instance of ScalarHandler.
     *
     * @param columnIndex The index of the column to retrieve from the
     * <code>ResultSet</code>.
     */
    public ScalarHandler(int columnIndex) {
        this(columnIndex, null);
    }

    /**
     * Creates a new instance of ScalarHandler.
     *
     * @param columnName The name of the column to retrieve from the
     * <code>ResultSet</code>.
     */
    public ScalarHandler(String columnName) {
        this(1, columnName);
    }

    /** Helper constructor
     * @param columnIndex The index of the column to retrieve from the
     * <code>ResultSet</code>.
     * @param columnName The name of the column to retrieve from the
     * <code>ResultSet</code>.
     */
    private ScalarHandler(int columnIndex, String columnName) {
        this.columnIndex = columnIndex;
        this.columnName = columnName;
    }

    /**
     * Returns one <code>ResultSet</code> column as an object via the
     * <code>ResultSet.getObject()</code> method that performs type
     * conversions.
     * @param rs <code>ResultSet</code> to process.
     * @return The column or <code>null</code> if there are no rows in
     * the <code>ResultSet</code>.
     *
     * @throws SQLException if a database access error occurs
     * @throws ClassCastException if the class datatype does not match the column type
     *
     * @see org.apache.commons.dbutils.ResultSetHandler#handle(java.sql.ResultSet)
     */
    // We assume that the user has picked the correct type to match the column
    // so getObject will return the appropriate type and the cast will succeed.
    @SuppressWarnings("unchecked")
    @Override
    public T handle(ResultSet rs) throws SQLException {

        if (rs.next()) {
        	//若是列名为null，那么就使用列码
            if (this.columnName == null) {
                return (T) rs.getObject(this.columnIndex);
            } else {
                return (T) rs.getObject(this.columnName);
            }

        } else {
            return null;
        }
    }
}

ScalarHandler用于获取单值，经常使用的是计算表的记录数count(*)。下面的代码是测试代码。用的数据库仍是以前的。数组

@Test
	public void testScalarHandler() throws Exception{
		String sql = "select * from t_user where id=?";
		Object[] params = new Object[]{3};
		ResultSetHandler<String> rsh = new ScalarHandler<String>(2);
		
		System.out.println(queryRunner.query(sql, rsh, params));
	}

这几个就是经常使用的Handler了。平时开发的话，通常是够用了。在查询方面，使用了策略模式。用户能够自定义ResultSetHandler，用户本身来实现对结果集的处理，也能够结合自定义的RowProcessor实现类来一块儿使用。安全