Creating classes dynamically with Java

问题:

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

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.

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

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

Is this possible and how would I do this?

EDIT: 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;
}

回答1:

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

• 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.

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.

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) {..}

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.

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

回答2:

There are many different ways to achieve this (e.g proxies, ASM), but the simplest approach, one that you can start with when prototyping is:

import java.io.*;
import java.util.*;
import java.lang.reflect.*;

public class MakeTodayClass {
  Date today = new Date();
  String todayMillis = Long.toString(today.getTime());
  String todayClass = "z_" + todayMillis;
  String todaySource = todayClass + ".java";

  public static void main (String args[]){
    MakeTodayClass mtc = new MakeTodayClass();
    mtc.createIt();
    if (mtc.compileIt()) {
       System.out.println("Running " + mtc.todayClass + ":\n\n");
       mtc.runIt();
       }
    else
       System.out.println(mtc.todaySource + " is bad.");
    }

  public void createIt() {
    try {
      FileWriter aWriter = new FileWriter(todaySource, true);
      aWriter.write("public class "+ todayClass + "{");
      aWriter.write(" public void doit() {");
      aWriter.write(" System.out.println(\""+todayMillis+"\");");
      aWriter.write(" }}\n");
      aWriter.flush();      
      aWriter.close();
      }
    catch(Exception e){
      e.printStackTrace();
      }
    }

  public boolean compileIt() {
    String [] source = { new String(todaySource)};
    ByteArrayOutputStream baos= new ByteArrayOutputStream();

    new sun.tools.javac.Main(baos,source[0]).compile(source);
    // if using JDK >= 1.3 then use
    //   public static int com.sun.tools.javac.Main.compile(source);    
    return (baos.toString().indexOf("error")==-1);
    }

  public void runIt() {
    try {
      Class params[] = {};
      Object paramsObj[] = {};
      Class thisClass = Class.forName(todayClass);
      Object iClass = thisClass.newInstance();
      Method thisMethod = thisClass.getDeclaredMethod("doit", params);
      thisMethod.invoke(iClass, paramsObj);
      }
    catch (Exception e) {
      e.printStackTrace();
      }
    }
}

回答3:

This is possible, but (I believe) you need something like ASM or BCEL.

Alternately, you could use something with more power (like Groovy).

回答4:

It will take a couple of minutes to create a data model class for each table, which you can easily map to the database with an ORM like Hibernate or by writing your own JDBC DAOs. It is far easier than delving deeply into reflection.

You could create a utility that interrogates the database structure for a table, and creates the data model class and DAO for you. Alternatively you could create the model in Java and create a utility to create the database schema and DAO from that (using reflection and Java 5 Annotations to assist). Don't forget that javaFieldNames are different from database_column_names typically.