在运行时创建简单的POJO类（字节码）（动态）

Question

我有以下情况..

我正在编写一些工具，针对数据库运行用户输入的查询并返回结果..

最简单的方法是将结果返回为：List<String[]>，但我需要更进一步。

我需要创建（在运行时）某些名称的POJO（或DTO），并为其创建字段和setter以及getter，并使用返回的数据填充它，然后将其返回给用户生成.class文件...

所以这里的想法是如何在运行时创建简单的类（字节码）（动态） 我做了一个基本的搜索，找到了很多lib including Apache BCEL但是我觉得我需要更简单的东西...

你怎么看？

感谢。

Answer 1

如果您使用CGLib，则可以轻松创建包含getter和setter的简单POJO：

public static Class<?> createBeanClass(
    /* fully qualified class name */
    final String className,
    /* bean properties, name -> type */
    final Map<String, Class<?>> properties){

    final BeanGenerator beanGenerator = new BeanGenerator();

    /* use our own hard coded class name instead of a real naming policy */
    beanGenerator.setNamingPolicy(new NamingPolicy(){
        @Override public String getClassName(final String prefix,
            final String source, final Object key, final Predicate names){
            return className;
        }});
    BeanGenerator.addProperties(beanGenerator, properties);
    return (Class<?>) beanGenerator.createClass();
}

测试代码：

public static void main(final String[] args) throws Exception{
    final Map<String, Class<?>> properties =
        new HashMap<String, Class<?>>();
    properties.put("foo", Integer.class);
    properties.put("bar", String.class);
    properties.put("baz", int[].class);

    final Class<?> beanClass =
        createBeanClass("some.ClassName", properties);
    System.out.println(beanClass);
    for(final Method method : beanClass.getDeclaredMethods()){
        System.out.println(method);
    }

}

<强>输出：

  

class some.ClassName
  public int [] some.ClassName.getBaz（）
  public void some.ClassName.setBaz（int []）
  public java.lang.Integer some.ClassName.getFoo（）
  public void some.ClassName.setFoo（java.lang.Integer）
  public java.lang.String some.ClassName.getBar（）
  public void some.ClassName.setBar（java.lang.String）

但问题是：你无法对这些方法进行编码，因为它们在编译时不存在，所以我不知道这对你有什么好处。

Answer 2

我过去曾使用过ASM。我喜欢的是ASMifier，它可以创建代码来生成类。例如我在Java代码中创建了一个通用POJO，其中包含Java中每种类型的一个字段，并使用ASMifier创建Java代码，以便从字节代码创建它，并将其用作模板来生成任意POJO。

正如@Michael建议的那样，您可能希望添加一种非反思的方式来获取任意字段。例如

public Set<String> fieldNames();
public Object getField(String name);
public void setField(String name, Object name);

你为什么要这样做？有些方法可以比使用常规地图更有效地使用Map<String, Object>样式对象。

另一种方法是使用Velocity生成Java源代码并使用Compiler API编译代码。使用它很痛苦所以我在这里编写了一个包装Essence JCF使用这种方法的唯一优点是可以轻松调试生成的代码。 （该库可以选择将java代码保存到调试器可以找到它的位置，以便当您单步执行生成的代码时）

Answer 3

那么This may also given a try。但如果有人能解释，我需要理解这一点。

更新：

想象一下，您的应用程序必须在运行时从某些外部配置动态创建Java POJO实例。使用字节码操作库之一可以轻松完成此任务。这篇文章演示了如何使用Javassist库完成此操作。

假设我们对动态创建的POJO应包含的属性进行了以下配置：

Map<String, Class<?>> props = new HashMap<String, Class<?>>();
props.put("foo", Integer.class);
props.put("bar", String.class);

让我们编写一个PojoGenerator，它动态生成给定类名的Class对象和一个包含所需属性的map：

import java.io.Serializable;
import java.util.Map;
import java.util.Map.Entry;

import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtField;
import javassist.CtMethod;
import javassist.NotFoundException;

public class PojoGenerator {

public static Class generate(String className, Map<String, Class<?>>  properties) throws NotFoundException,
        CannotCompileException {

    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.makeClass(className);

    // add this to define a super class to extend
    // cc.setSuperclass(resolveCtClass(MySuperClass.class));

    // add this to define an interface to implement
    cc.addInterface(resolveCtClass(Serializable.class));

    for (Entry<String, Class<?>> entry : properties.entrySet()) {

        cc.addField(new CtField(resolveCtClass(entry.getValue()), entry.getKey(), cc));

        // add getter
        cc.addMethod(generateGetter(cc, entry.getKey(), entry.getValue()));

        // add setter
        cc.addMethod(generateSetter(cc, entry.getKey(), entry.getValue()));
    }

    return cc.toClass();
}

private static CtMethod generateGetter(CtClass declaringClass, String fieldName, Class fieldClass)
        throws CannotCompileException {

    String getterName = "get" + fieldName.substring(0, 1).toUpperCase()
            + fieldName.substring(1);

    StringBuffer sb = new StringBuffer();
    sb.append("public ").append(fieldClass.getName()).append(" ")
            .append(getterName).append("(){").append("return this.")
            .append(fieldName).append(";").append("}");
    return CtMethod.make(sb.toString(), declaringClass);
}

private static CtMethod generateSetter(CtClass declaringClass, String fieldName, Class fieldClass)
        throws CannotCompileException {

    String setterName = "set" + fieldName.substring(0, 1).toUpperCase()
            + fieldName.substring(1);

    StringBuffer sb = new StringBuffer();
    sb.append("public void ").append(setterName).append("(")
            .append(fieldClass.getName()).append(" ").append(fieldName)
            .append(")").append("{").append("this.").append(fieldName)
            .append("=").append(fieldName).append(";").append("}");
    return CtMethod.make(sb.toString(), declaringClass);
}

private static CtClass resolveCtClass(Class clazz) throws NotFoundException {
    ClassPool pool = ClassPool.getDefault();
    return pool.get(clazz.getName());
}
}

就是这样！

使用PojoGenerator非常简单。让我们生成一些POJO，通过反射输出所有方法，设置然后得到一些属性：

public static void main(String[] args) throws Exception {

Map<String, Class<?>> props = new HashMap<String, Class<?>>();
props.put("foo", Integer.class);
props.put("bar", String.class);

Class<?> clazz = PojoGenerator.generate(
        "net.javaforge.blog.javassist.Pojo$Generated", props);

Object obj = clazz.newInstance();

System.out.println("Clazz: " + clazz);
System.out.println("Object: " + obj);
System.out.println("Serializable? " + (obj instanceof Serializable));

for (final Method method : clazz.getDeclaredMethods()) {
    System.out.println(method);
}

// set property "bar"
clazz.getMethod("setBar", String.class).invoke(obj, "Hello World!");

// get property "bar"
String result = (String) clazz.getMethod("getBar").invoke(obj);
System.out.println("Value for bar: " + result);

}

执行以上操作将导致以下控制台输出：

Clazz: class net.javaforge.blog.javassist.Pojo$Generated
Object: net.javaforge.blog.javassist.Pojo$Generated@55571e
Serializable? true
public void     net.javaforge.blog.javassist.Pojo$Generated.setBar(java.lang.String)
public java.lang.String     net.javaforge.blog.javassist.Pojo$Generated.getBar()
public java.lang.Integer     net.javaforge.blog.javassist.Pojo$Generated.getFoo()
public void     net.javaforge.blog.javassist.Pojo$Generated.setFoo(java.lang.Integer)
Value for bar: Hello World!

Answer 4

调用者做什么与动态生成的类以及他的代码因此无法知道的类？访问它的唯一方法是通过反射。返回List<String[]>或Map<String, String>实际上是一种更清洁，更实用的设计。

Answer 5

我也讨厌写getter和setter。我更喜欢使用POJO，甚至是声明为嵌套类的POJO。

还有另一种方法可以做到这一点，即使使用旧的服务器和技术，也没有引入Springs（我们使用JBoss 4.2和JBoss不完整的EJB 3.0）。扩展org.apache.commons.beanutils.BeanMap，您可以将POJO包装在bean映射中，当您获取或放置时，可以使用反射操作字段。如果getter或setter不存在，我们只需使用字段操作来获取它。显然它不是真正的豆子，所以这完全没问题。

package com.aaa.ejb.common;

import java.io.Serializable;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.HashSet;
import java.util.Set;

import org.apache.commons.beanutils.BeanMap;
import org.apache.commons.collections.set.UnmodifiableSet;
import org.apache.log4j.Logger;

/**
 * I want the bean map to be able to handle a POJO. 
 * @author gbishop
 */
public final class NoGetterBeanMap extends BeanMap {

    private static final Logger LOG = Logger.getLogger(NoGetterBeanMap.class);

    /**
     * Gets a bean map that can handle writing to a pojo with no getters or setters.
     * @param bean
     */
    public NoGetterBeanMap(Object bean) {
        super(bean);
    }

    /* (non-Javadoc)
     * @see org.apache.commons.beanutils.BeanMap#get(java.lang.Object)
     */
    public Object get(Object name) {
        Object bean = getBean();
        if ( bean != null ) {
            Method method = getReadMethod( name );
            if ( method != null ) {
                try {
                    return method.invoke( bean, NULL_ARGUMENTS );
                }
                catch (  IllegalAccessException e ) {
                    logWarn( e );
                }
                catch ( IllegalArgumentException e ) {
                    logWarn(  e );
                }
                catch ( InvocationTargetException e ) {
                    logWarn(  e );
                }
                catch ( NullPointerException e ) {
                    logWarn(  e );
                }
            } else {
                if(name instanceof String) {
                    Class<?> c = bean.getClass();
                    try {
                        Field datafield = c.getDeclaredField( (String)name );
                        datafield.setAccessible(true);
                        return datafield.get(bean);
                    } catch (SecurityException e) {
                        throw new IllegalArgumentException( e.getMessage() );
                    } catch (NoSuchFieldException e) {
                        throw new IllegalArgumentException( e.getMessage() );
                    } catch (IllegalAccessException e) {
                        throw new IllegalArgumentException( e.getMessage() );
                    }
                }
            }
        }
        return null;
    }

    /* (non-Javadoc)
     * @see org.apache.commons.beanutils.BeanMap#put(java.lang.Object, java.lang.Object)
     */
    public Object put(Object name, Object value) throws IllegalArgumentException, ClassCastException {
        Object bean = getBean();
        if ( bean != null ) {
            Object oldValue = get( name );
            Method method = getWriteMethod( name );
            Object newValue = null;
            if ( method == null ) {
                if(name instanceof String) {//I'm going to try setting the property directly on the bean.
                    Class<?> c = bean.getClass();
                    try {
                        Field datafield = c.getDeclaredField( (String)name );
                        datafield.setAccessible(true);
                        datafield.set(bean, value);
                        newValue = datafield.get(bean);
                    } catch (SecurityException e) {
                        throw new IllegalArgumentException( e.getMessage() );
                    } catch (NoSuchFieldException e) {
                        throw new IllegalArgumentException( e.getMessage() );
                    } catch (IllegalAccessException e) {
                        throw new IllegalArgumentException( e.getMessage() );
                    }
                } else {
                    throw new IllegalArgumentException( "The bean of type: "+ 
                            bean.getClass().getName() + " has no property called: " + name );
                }
            } else {
                try {
                    Object[] arguments = createWriteMethodArguments( method, value );
                    method.invoke( bean, arguments );
                    newValue = get( name );
                } catch ( InvocationTargetException e ) {
                    logInfo( e );
                    throw new IllegalArgumentException( e.getMessage() );
                } catch ( IllegalAccessException e ) {
                    logInfo( e );
                    throw new IllegalArgumentException( e.getMessage() );
                }
                firePropertyChange( name, oldValue, newValue );
            }
            return oldValue;
        }
        return null;
    }

    /* (non-Javadoc)
     * @see org.apache.commons.beanutils.BeanMap#keySet()
     */
    public Set keySet() {
        Class<?> c = getBean().getClass();
        Field[] fields = c.getDeclaredFields();
        Set<String> keySet = new HashSet<String>(super.keySet());
        for(Field f: fields){
            if( Modifier.isPublic(f.getModifiers()) && !keySet.contains(f.getName())){
                keySet.add(f.getName());
            }
        }
        keySet.remove("class");
        return UnmodifiableSet.decorate(keySet);
    }
}

棘手的部分是让POJO退回，但反思可以帮助你：

/**
* Returns a new instance of the specified object.  If the object is a bean, 
* (serializable, with a default zero argument constructor), the default 
* constructor is called.  If the object is a Cloneable, it is cloned, if the
* object is a POJO or a nested POJO, it is cloned using the default 
* zero argument constructor through reflection.  Such objects should only be
* used as transfer objects since their constructors and initialization code
* (if any) have not have been called. 
* @param obj
* @return A new copy of the object, it's fields are blank.
*/
public static Object constructBeanOrPOJO(final Object obj) {

    Constructor<?> ctor = null;
    Object retval = null;
    //Try to invoke where it's Serializable and has a public zero argument constructor.
    if(obj instanceof Serializable){
        try {
            ctor = obj.getClass().getConstructor((Class<?>)null);
            if(ctor.isAccessible()){
                retval = ctor.newInstance();
                //LOG.info("Serializable class called with a public constructor.");
                return retval;
            }
        } catch (Exception ignoredTryConeable) {
        }
    }
    //Maybe it's Clonable.
    if(obj instanceof Cloneable){
        try {
            Method clone = obj.getClass().getMethod("clone");
            clone.setAccessible(true);
            retval = clone.invoke(obj);
            //LOG.info("Cloneable class called.");
            return retval;
        } catch (Exception ignoredTryUnNestedClass) {
        }
    }
    try {
        //Maybe it's not a nested class.
        ctor = obj.getClass().getDeclaredConstructor((Class<?>)null);
        ctor.setAccessible(true);
        retval = ctor.newInstance();
        //LOG.info("Class called with no public constructor.");
        return retval; 
    } catch (Exception ignoredTryNestedClass) {
    }
    try {
       Constructor[] cs = obj.getClass().getDeclaredConstructors();
       for(Constructor<?> c: cs){
          if(c.getTypeParameters().length==0){
             ctor = c;
             ctor.setAccessible(true);
             retval = ctor.newInstance();
             return retval;
          }
       }
       //Try a nested class class.
       Field parent = obj.getClass().getDeclaredField("this$0");
       parent.setAccessible(true);
       Object outer = (Object) parent.get(obj);
       //ctor = (Constructor<? extends Object>) obj.getClass().getConstructors()[0];//NO, getDECLAREDConstructors!!!
       ctor = (Constructor<? extends Object>)         obj.getClass().getDeclaredConstructor(parent.get(obj).getClass());
       ctor.setAccessible(true);
       retval = ctor.newInstance(outer);
       //LOG.info("Nested class called with no public constructor.");
       return retval;
    } catch (Exception failure) {
        throw new IllegalArgumentException(failure);
    }
}

从bean，cloneable或POJO获取泛型bean的示例代码：

public List<Object> getGenericEJBData(String tableName, Object desiredFields, Object beanCriteria){
    NoGetterBeanMap desiredFieldMap = new NoGetterBeanMap(desiredFields);
    NoGetterBeanMap criteriaMap = new NoGetterBeanMap(beanCriteria);
    List<Object> data = new ArrayList<Object>();
    List<Map<String, Object>> mapData = getGenericEJBData(tableName, desiredFieldMap, criteriaMap);
    for (Map<String,Object> row: mapData) {
        Object bean = NoGetterBeanMap.constructBeanOrPOJO(desiredFields);//Cool eh?
        new NoGetterBeanMap(bean).putAll(row);//Put the data back in too!
        data.add(bean);
    }
    return data;
}

EJB的使用示例：

IGenericBean genericRemote = BeanLocator.lookup(IGenericBean.class);

//This is the minimum required typing.
class DesiredDataPOJO {
    public String makename="";//Name matches column and return type. 
}
class CriteriaPOJO {
    //Names match column and contains criteria values.
    public String modelname=model,yearid=year; 
}

List<DesiredDataPOJO> data = 
    genericRemote.getGenericEJBData(ACES_VEHICLE_TABLE, new DesiredDataPOJO(), new CriteriaPOJO() );
for (DesiredDataPOJO o: data) {
    makes.add(o.makename);
}

EJB有这样的接口：

package com.aaa.ejb.common.interfaces;

import java.util.List;
import java.util.Map;

import javax.ejb.Local;
import javax.ejb.Remote;

/**
 * @see
 * http://trycatchfinally.blogspot.com/2006/03/remote-or-local-interface.html
 * 
 * Note that the local and remote interfaces extend a common business interface.
 * Also note that the local and remote interfaces are nested within the business
 * interface. I like this model because it reduces the clutter, keeps related
 * interfaces together, and eases understanding.
 * 
 * When using dependency injection, you can specify explicitly whether you want 
 * the remote or local interface.  For example:
 * @EJB(beanInterface=services.DistrictService.IRemote.class)
 * public final void setDistrictService(DistrictService districtService) {
 *    this.districtService = districtService;
 * }
 */
public interface IGenericBean {

    @Remote 
    public interface IRemote extends IGenericBean {
    }

    @Local 
    public interface ILocal extends IGenericBean {
    }

    /**
     * Gets a list of beans containing data.
     * Requires a table name and pair of beans containing the fields
     * to return and the criteria to use.
     * 
     * You can even use anonymous inner classes for the criteria.
     * EX: new Object() { public String modelname=model,yearid=year; }
     * 
     * @param tableName
     * @param fields
     * @param criteria
     * @return
     */
    public <DesiredFields> List<DesiredFields> getGenericEJBData(String tableName, DesiredFields desiredFields, Object beanCriteria);

}

你可以想象ejb的实现是什么样的，现在我们正在构建预准备语句并调用它们但是我们可以使用标准，或者像hibernate那样更酷的东西或者任何我想要的东西。

这是一个粗略的例子（有些人不喜欢这部分）。在示例中，我们有一个表格，其中包含第三范式的数据。为此，bean字段必须与表列名匹配。 toLowerCase（）调用以防万一这是一个正在使用的REAL bean，这将搞砸名称匹配（MyField与getMyfield）。这部分可能会更好一点。特别是顺序和不同应该是标志或其他东西。可能还会出现其他边缘条件。当然我只需要编写这个ONCE，并且为了性能，没有什么能阻止你有更精确的数据接收器来提高性能。

@Stateless
public class GenericBean implements ILocal, IRemote {

...

/* (non-Javadoc)
 * @see com.aaa.ejb.acesvehicle.beans.interfaces.IAcesVehicleBean#getGenericEJBData(java.lang.String, java.util.Map, java.util.Map)
 */
@Override
public List<Map<String, Object>> getGenericEJBData(String tableName,Map<String, Object> desiredFields, Map<String, Object> criteria){
    try {
        List<Map<String,Object>> dataFieldKeyValuePairs = new ArrayList<Map<String,Object>>();

        StringBuilder sql = new StringBuilder("SELECT DISTINCT ");
        int selectDistinctLength = sql.length();
        for(Object key : desiredFields.keySet()){
            if(desiredFields.get(key)!=null) {
                sql.append(key).append(", ");
            }
        }
        sql.setLength(sql.length()-2);//Remove last COMMA.
        int fieldsLength = sql.length();
        sql.append(" FROM ").append(tableName).append(" WHERE ");

        String sep = "";//I like this, I like it a lot.
        for(Object key : criteria.keySet()){
            sql.append(sep);
            sql.append(key).append(" = COALESCE(?,").append(key).append(") ");
            sep = "AND ";
        }
        sql.append(" ORDER BY ").append(sql.substring(selectDistinctLength, fieldsLength));

        PreparedStatement ps = connection.prepareStatement(sql.toString(), ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_READ_ONLY);
        int criteriaCounter=1;
        for(Object key : criteria.keySet()){
            ps.setObject(criteriaCounter++, criteria.get(key));
        }
        ResultSet rs = ps.executeQuery();
        while (rs.next()) {
            Map<String,Object> data = new HashMap<String,Object>();
            int columnIndex = rs.getMetaData().getColumnCount();
            for(int x=0;x<columnIndex;x++){
                String columnName = rs.getMetaData().getColumnName(x+1);
                if(desiredFields.keySet().contains(columnName.toLowerCase())){
                    //Handle bean getters and setters with different case than metadata case.
                    data.put(columnName.toLowerCase(), rs.getObject(x+1));
                } else {
                    data.put(columnName, rs.getObject(x+1));
                }
            }
            dataFieldKeyValuePairs.add(data);
        }
        rs.close();
        ps.close();
        return dataFieldKeyValuePairs;
    } catch (SQLException sqle) {
        LOG.debug("National database access failed.", sqle);
        throw new EJBException(new DataSourceException("Database access failed. \n"
                + "getGenericEJBData()", sqle.getMessage()));
    }

}

Answer 6

解决方案由以下链接的第一个答案提供，这是Stack的问题之一）溢出本身。 Dynamically create table and java classes at runtime