从JSON (Gson)反序列化时如何初始化PropertyChangeSupport？

Question

我的对象实现了PropertyChangeSupport，但是当我从json字符串反序列化时，变量propertyChangeSupport将是null，尽管我自己在默认构造函数中使用new PropertyChangeSupport(this)初始化了该值。如何使用Gson正确地初始化或反序列化它？

假设我有一个目标：

public class Blah implements BlahInterface {
    private PropertyChangeSupport propertyChangeSupport;

    protected int id;
    protected BlahType type;

    public Blah() {
        propertyChangeSupport = new PropertyChangeSupport(this);
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public BlahType getType() {
        return type;
    }

    public void setType(BlahType type) {
        this.type = type;
    }

    public void addPropertyChangeListener(PropertyChangeListener listener) {
        this.propertyChangeSupport.addPropertyChangeListener(listener);
    }

    public PropertyChangeListener[] getPropertyChangeListeners() {
        return this.propertyChangeSupport.getPropertyChangeListeners();
    }
}

我也尝试把new PropertyChangeSupport(this);直接放在开始，也是不去。我想避免手动创建一个函数，比如initializePropertyChangeSupport()，然后在反序列化之后手动调用它，因为这有点难看。

我想做的是：

JsonArray ja = json.get("blahs").getAsJsonArray();
ja.forEach(item -> {
    Blah blah = BlahInterface.Parse(item.toString());
    // But here I can't addPropertyChangeListener because propertyChangeSupport is null
    // vvvvvvvvvvvv
    blah.addPropertyChangeListener(new PropertyChangeListener() {

        @Override
        public void propertyChange(PropertyChangeEvent evt) {
            BlahState state = (BlahState) evt.getNewValue();
            Logger.debug("Property had been updated, " + state.toString());
        }
    });
});

这是我的json解析函数：

@SuppressWarnings("unchecked")
public static <T extends Blah> T Parse(String json) {
    Gson gson = new Gson();

    Blah t = new Blah(gson.fromJson(json, Blah.class));
    switch (t.getType()) {
        case blahone:
            return (T) gson.fromJson(json, BlahOne.class);
        default:
            return (T) t;
    }
};

Answer 1

这个问题的解决方案是在我的对象中实现InstanceCreator<T>。这样，当Gson试图反序列化对象时，它将调用createInstance函数，该函数依次返回一个带有初始化PropertyChangeSupport变量的适当对象。下面是示例代码：

public class Blah implements InstanceCreator<Blah> {
    private final transient PropertyChangeSupport pcs = new PropertyChangeSupport(this);

    ...

    public void addPropertyChangeListener(PropertyChangeListener listener) {
        this.pcs.addPropertyChangeListener(listener);
    }

    public void removePropertyChangeListener(PropertyChangeListener listener) {
        this.pcs.removePropertyChangeListener(listener);
    }

    @Override
    public Blah createInstance(Type type) {
        return new Blah();
    }   
}

注意：pcs上存在瞬态关键字，这样Gson就可以在序列化期间跳过它，否则Gson会抛出异常。