JSF 使用ValueBinding類

3.6  使用ValueBinding類

javax.faces.el.ValueBinding類表示一個對象，可以用它來訪問由一個動作或值引用表達式所表示的屬性。可調用javax.faces.application.Application類的getValueBinding方法，同時傳入值引用，以此來獲取一個ValueBinding的實例。

ValueBinding類有4個方法：getType、getValue、setValue和isReadOnly。我們在以下的章節中討論這些方法。

3.6.1  獲取ValueBinding所表示的對象的類型

調用Application類的getValueBinding方法需要傳入一個值引用，返回值可能是一個JavaBean、一個JavaBean的屬性或是另外一個對象。getType方法可返回ValueBinding所表示的對象的類型。下面是getType方法的簽名：

public abstract Class getType(FacesContext facesContext)

throws javax.faces.el.PropertyNotFoundExcetpion;

比如，假設您的應用配置文件裏有如下managed-bean標記：

<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>

則使用以下代碼會在控制檯輸出表示ShoppingCartBean的類名：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  獲取和設置ValueBinding對象的屬性

要獲取由ValueBinding對象表示的屬性值，可使用getValue方法。此方法具有如下簽名：

public abstract Object getValue(FacesContext facesContext)

throws javax.faces.el.PropertyNotFoundException

比如，下面的代碼提取由一個ValueBinding表示的ShoppingCartBean對象：

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方法則是用來改變由ValueBinding對象表示的屬性值。其方法簽名如下：

public abstract void setValue

(FacesContext facesContext, Object value)

throws javax.faces.el.PropertyNotFoundException

比如，假設ShoppingCartBean有一個叫作purchaseId的屬性，其聲明如下：

private String purchaseId;

public String getPurchaseId() {

return purchaseId;

}

public void setPurchaseId(String purchaseId) {

this.purchaseId = purchaseId;

}

則下面的代碼可獲取ShoppingCartBean對象的purchaseId屬性，並將其值設爲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));

這段代碼會在控制檯輸出如下文本：

Purchase Id: 12345

3.6.3  檢查ValueBinding屬性是否可寫

isReadOnly方法返回一個指示當前ValueBinding對象所表示的屬性是否可寫的boolean值。其方法簽名如下：

public abstract boolean isReadOnly(FacesContext facesContext)

throws javax.faces.el.PropertyNotFoundException

比如，下面的代碼在改變ShoppingCartBean的purchaseId屬性前先檢查其是否可寫。

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");