使用JavaBeans在类之间传递数据

Question

A Piece of important information: The classes are all separate files and there are about 10 beans in total .

I am working on a project with multiple classes through which data must be passed. Ie a couple strings from say Class1 must be able to be used in Class2.

My program uses JavaBeans with set and get methods but if I set a bean in one class and try to get that data in another class I just get a null value returned.

I am unsure as to what the best method is, I have the beans nicely defined and would like to keep using them but I do not know how.

Could someone point me in the correct direction or give an example. Thanks

Edit

Bean class snippet;

public class beans implements java.io.Serializable {



private String string1;


public String getstring1() {

    return this.string1;
}

public void setstring1(String string1) {
    this.string1 = string1;
}

And the setter code in say class1:

beans bean = new beans();   
bean.setstring1("test");

And the class where the bet is "got", class2

beans bean = new beans(); String text = bean.getstring1();

That is pretty much how I am using the beans and they are not working as I want them to.

Answer 1

In your example you are creating a new bean in each class. The first and the second class have references to two different objects, that's why they can't access the same values.

There are multiple ways to solve this and it really depends on your application. But let me suggest one generic solution.

You can create a BeanRepository which is responsible for having references to all bean objects. Other classes then need to know the id of the bean and they can get a reference.

BeanRepository (notice the static methods):

public class BeanRepository {
    private static Map<Integer, Bean> beanMap = new HashMap<Integer, Bean>();

    public static void putBean(int id, Bean bean) {
        beanMap.put(id, bean);
    }

    public static Bean getBean(int id) {
        return beanMap.get(id);
    }
}

The bean:

public class Bean {
    private String name;

    public Bean(String name) {
        this.name = name;
    }

    public String whoAmI() {
        return name;
    }
}

Classes A and B:

public class ClassA {
    private Bean bean;

    public ClassA(int beanId) {
        this.bean = BeanRepository.getBean(beanId);
    }

    public void test() {
        System.out.println("I am ClassA. You are " + bean.whoAmI());
    }
}

public class ClassB {
    private Bean bean;

    public ClassB(int beanId) {
        this.bean = BeanRepository.getBean(beanId);
    }

    public void test() {
        System.out.println("I am ClassB. You are " + bean.whoAmI());
    }
}

Test method:

public class Main {
    public static void main(String[] args) {
        BeanRepository.putBean(1, new Bean("one"));

        ClassA a = new ClassA(1);
        ClassB b = new ClassB(1);

        a.test();
        b.test();
    }
}

Answer 2

container I am very puzzled by your question. Are you referring to Enterprise Java Beans?

If you just mean ordinary JavaBeans just featuring get and set methods, the first thing I would advise is to learn how to use jUnit . Test your data bean first inorder to ensure it is working as intended.

Beside that if your class live within the same JVA (you only start a single java.exe / run a single application) everything should just work fine.

public class Data { 
    private String value; 
    public void set(String value) { this.value = value; }
    public String get() { return this.value; }
}

public class ClassA {
    Data data = new Data();
    ClassA() { data.set("From ClassA"); }
}

public class ClassB {
    Data data;
    ClassB(Data data) { this.data = data; }
    public void print() {
      System.out.println(data.get());
    }
}

public static void main(String args []) {
    new ClassB(new ClassA().data).println();
}

This is a simple example using a data object to pass informations around.

Hope this is what you want to know.

[Update]

After you add some code I see the problem. As other users already wrote you are creating new objects (instances) every time. Since the string1 is a property of that class ever instance of it will have their own value.

The default example is a person. Creating a Person class with a property name (similar to your string1) you can create two persons (instances of class Person). You can now name every person individual.

Thats what Class mean. You specify the properties (instance variables) and behavior (methods) of instances (actual object) of that class.

If you want to share information you need a way to pass(!) an instance (object) of a Class to other instances. In Java different way exist. You can use static variables which are global (bound to the Class instead of an instance of a class), you can use singleton pattern (which employes the static variable), you can use ThreadLocal (a global store limited to the current thread), you may use managers / repositories storing objects and you pass the manager / repository objects along, you can use databases or you can use a dependency injector which is like a transparent object manager. Those are basically it.

For your use case I would use Singleton first.

public class MySingleton {
   private static Bean myBean;
   public static void setBean(Bean myBean) { MySingleton.myBean = myBean; }
   public static Bean getBean() { return myBean; }
}

Using the class is straight forward:

Bean bean = new Bean();
bean.setString1("test");
MySingleton.setBean(bean);
Bean bean2 = MySingleton.getBean();
System.out.println(bean2.getString1()); //prints test

But beware this is the lazy way of doing things. Using static has some draw backs if you have a complex project especially when it comes to serialization, concurrency and reuse.

Answer 3

If you'se serializing/deserializing your classes, make sure that

1. string fields themselves are not marked as transient.

2. you're not using custom writeObject(ObjectOutputStream ) methods in your object where you forgot your string fields.

3. you're not cloning your class with custom clone() methods where you forgot your string field

4. you wrote setter and getter properly in the first place.

and things shoud work ;)