使用Java动态创建类 [英] Creating classes dynamically with Java

问题描述

我试图找到有关这方面的信息，但空手而归：

I have tried to find information about this but have come up empty handed:

我认为可以使用反射或代理在Java中动态创建一个类但是我不知道怎么做。我正在实现一个简单的数据库框架，我使用反射创建SQL查询。该方法获取具有数据库字段作为参数的对象，并基于该创建查询。但是如果我也可以动态创建对象本身就非常有用，所以我不需要为每个表都有一个简单的数据包装器对象。

I gather it is possible to create a class dynamically in Java using reflection or proxies but I can't find out how. I'm implementing a simple database framework where I create the SQL queries using reflection. The method gets the object with the database fields as a parameter and creates the query based on that. But it would be very useful if I could also create the object itself dynamically so I wouldn't have the need to have a simple data wrapper object for each table.

动态类只需要简单的字段（ String ， Integer ， Double ），例如

The dynamic classes would only need simple fields (String, Integer, Double), e.g.

public class Data {
  public Integer id;
  public String name;
}

这可行，我该怎么做？

编辑：这是我如何使用它：

This is how I would use this:

/** Creates an SQL query for updating a row's values in the database.
 *
 * @param entity Table name.
 * @param toUpdate Fields and values to update. All of the fields will be
 * updated, so each field must have a meaningful value!
 * @param idFields Fields used to identify the row(s).
 * @param ids Id values for id fields. Values must be in the same order as
 * the fields.
 * @return
 */
@Override
public String updateItem(String entity, Object toUpdate, String[] idFields,
        String[] ids) {
    StringBuilder sb = new StringBuilder();

    sb.append("UPDATE ");
    sb.append(entity);
    sb.append("SET ");

    for (Field f: toUpdate.getClass().getDeclaredFields()) {
        String fieldName = f.getName();
        String value = new String();
        sb.append(fieldName);
        sb.append("=");
        sb.append(formatValue(f));
        sb.append(",");
    }

    /* Remove last comma */
    sb.deleteCharAt(sb.toString().length()-1);

    /* Add where clause */
    sb.append(createWhereClause(idFields, ids));

    return sb.toString();
}
 /** Formats a value for an sql query.
 *
 * This function assumes that the field type is equivalent to the field
 * in the database. In practice this means that this field support two
 * types of fields: string (varchar) and numeric.
 *
 * A string type field will be escaped with single parenthesis (') because
 * SQL databases expect that. Numbers are returned as-is.
 *
 * If the field is null, a string containing "NULL" is returned instead.
 * 
 * @param f The field where the value is.
 * @return Formatted value.
 */
String formatValue(Field f) {
    String retval = null;
    String type = f.getClass().getName();
    if (type.equals("String")) {
        try {
            String value = (String)f.get(f);
            if (value != null) {
                retval = "'" + value + "'";
            } else {
                retval = "NULL";
            }
        } catch (Exception e) {
            System.err.println("No such field: " + e.getMessage());
        }
    } else if (type.equals("Integer")) {
        try {
            Integer value = (Integer)f.get(f);
            if (value != null) {
                retval = String.valueOf(value);
            } else {
                retval = "NULL";
            }
        } catch (Exception e) {
            System.err.println("No such field: " + e.getMessage());
        }
    } else {
        try {
            String value = (String) f.get(f);
            if (value != null) {
                retval = value;
            } else {
                retval = "NULL";
            }
        } catch (Exception e) {
            System.err.println("No such field: " + e.getMessage());
        }
    }
    return retval;
}

推荐答案

可以生成课程（通过 cglib ， asm ， javassist ， bcel ），但你不应该这样做。为什么？

It is possible to generate classes (via cglib, asm, javassist, bcel), but you shouldn't do it that way. Why?

• 使用该库的代码应该是类型 Object 并获得所有使用反射的字段 - 不是一个好主意


• java是静态类型语言，你想引入动态类型 - 它不是那个地方。

• the code that's using the library should expect type Object and get all the fields using reflection - not a good idea
• java is statically typed language, and you want to introduce dynamic typing - it's not the place.

如果您只是希望数据采用未定义的格式，那么您可以在数组中返回它，例如 Object [] ，或者 Map< String，Object> 如果你想要它们命名，并从那里得到它 - 它将为你不必要的类生成节省很多麻烦，仅用于包含一些数据这将通过反射获得。

If you simply want the data in an undefined format, then you can return it in an array, like Object[], or Map<String, Object> if you want them named, and get it from there - it will save you much trouble with unneeded class generation for the only purpose of containing some data that will be obtained by reflection.

你可以做的是拥有预定义的类来保存数据，并将它们作为参数传递给查询方法。例如：

What you can do instead is have predefined classes that will hold the data, and pass them as arguments to querying methods. For example:

 public <T> T executeQuery(Class<T> expectedResultClass, 
      String someArg, Object.. otherArgs) {..}

因此，您可以对传递的 expectedResultClass 使用反射来创建该类型的新对象，并使用查询结果填充它。

Thus you can use reflection on the passed expectedResultClass to create a new object of that type and populate it with the result of the query.

那就是说，我认为你可以使用现有的东西，比如ORM框架（Hibernate，EclipseLink），spring的 JdbcTemplate 等。

That said, I think you could use something existing, like an ORM framework (Hibernate, EclipseLink), spring's JdbcTemplate, etc.

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