JSF 使用ValueBinding类

3.6  使用ValueBinding类

javax.faces.el.ValueBinding类表示一个对象，可以用它来访问由一个动作或值引用表达式所表示的属性。可调用javax.faces.application.Application类的getValueBinding方法，同时传入值引用，以此来获取一个ValueBinding的实例。

ValueBinding类有4个方法：getType、getValue、setValue和isReadOnly。我们在以下的章节中讨论这些方法。

3.6.1  获取ValueBinding所表示的对象的类型

调用Application类的getValueBinding方法需要传入一个值引用，返回值可能是一个JavaBean、一个JavaBean的属性或是另外一个对象。getType方法可返回ValueBinding所表示的对象的类型。下面是getType方法的签名：

public abstract Class getType(FacesContext facesContext)

throws javax.faces.el.PropertyNotFoundExcetpion;

比如，假设您的应用配置文件里有如下managed-bean标记：

<managed-bean>

<managed-bean-name>shoppingCartBean</managed-bean-name>

<managed-bean-class>

buydirect.ShoppingCartBean

</managed-bean-class>

<managed-bean-scope>session</managed-bean-scope>

</managed-bean>

则使用以下代码会在控制台输出表示ShoppingCartBean的类名：buydirect.ShoppingCartBean。

FacesContext facesContext = FacesContext.getCurrentInstance();

ApplicationFactory factory = (ApplicationFactory)

FactoryFinder.getFactory(FactoryFinder.APPLICATION_FACTORY);

Application application = factory.getApplication();

ValueBinding valueBinding =

application.getValueBinding("shoppingCartBean");

System.out.println(valueBinding.getType(facesContext).getName());

3.6.2  获取和设置ValueBinding对象的属性

要获取由ValueBinding对象表示的属性值，可使用getValue方法。此方法具有如下签名：

public abstract Object getValue(FacesContext facesContext)

throws javax.faces.el.PropertyNotFoundException

比如，下面的代码提取由一个ValueBinding表示的ShoppingCartBean对象：

FacesContext facesContext = FacesContext.getCurrentInstance();

ApplicationFactory factory = (ApplicationFactory)

FactoryFinder.getFactory(FactoryFinder.APPLICATION_FACTORY);

Application application = factory.getApplication();

ValueBinding valueBinding =

application.getValueBinding("shoppingCartBean");

ShoppingCartBean bean =

(ShoppingCartBean) valueBinding.getValue(facesContext);

setValue方法则是用来改变由ValueBinding对象表示的属性值。其方法签名如下：

public abstract void setValue

(FacesContext facesContext, Object value)

throws javax.faces.el.PropertyNotFoundException

比如，假设ShoppingCartBean有一个叫作purchaseId的属性，其声明如下：

private String purchaseId;

public String getPurchaseId() {

return purchaseId;

}

public void setPurchaseId(String purchaseId) {

this.purchaseId = purchaseId;

}

则下面的代码可获取ShoppingCartBean对象的purchaseId属性，并将其值设为12345。

FacesContext facesContext = FacesContext.getCurrentInstance();

ApplicationFactory factory = (ApplicationFactory)

FactoryFinder.getFactory(FactoryFinder.APPLICATION_FACTORY);

Application application = factory.getApplication();

ValueBinding valueBinding =

application.getValueBinding("ShoppingCartBean.purchaseId");

valueBinding.setValue(facesContext, "12345");

// print the current value

System.out.println("Purchase Id:" +

valueBinding.getValue(facesContext));

这段代码会在控制台输出如下文本：

Purchase Id: 12345

3.6.3  检查ValueBinding属性是否可写

isReadOnly方法返回一个指示当前ValueBinding对象所表示的属性是否可写的boolean值。其方法签名如下：

public abstract boolean isReadOnly(FacesContext facesContext)

throws javax.faces.el.PropertyNotFoundException

比如，下面的代码在改变ShoppingCartBean的purchaseId属性前先检查其是否可写。

FacesContext facesContext = FacesContext.getCurrentInstance();

ApplicationFactory factory = (ApplicationFactory)

FactoryFinder.getFactory(FactoryFinder.APPLICATION_FACTORY);

Application application = factory.getApplication();

ValueBinding valueBinding =

application.getValueBinding("shoppingCartBean.purchaseId");

if (!valueBinding.isReadOnly(facesContext))

valueBinding.setValue(facesContext, "12345");