1 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。
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3 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。
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5 3.6.1 获取ValueBinding所表示的对象的类型
6 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：
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8 public abstract Class getType(FacesContext facesContext)
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10 throws javax.faces.el.PropertyNotFoundExcetpion;
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12 比如，假设您的应用配置文件里有如下managed-bean标记：
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16 shoppingCartBean
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18 class>
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20 buydirect.ShoppingCartBean
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22 class>
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24 session
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28 则使用以下代码会在控制台输出表示ShoppingCartBean的类名：buydirect.ShoppingCartBean。
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30 FacesContext facesContext = FacesContext.getCurrentInstance();
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32 ApplicationFactory factory = (ApplicationFactory)
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34 FactoryFinder.getFactory(FactoryFinder.APPLICATION_FACTORY);
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36 Application application = factory.getApplication();
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38 ValueBinding valueBinding =
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40 application.getValueBinding("shoppingCartBean");
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42 System.out.println(valueBinding.getType(facesContext).getName());
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44 3.6.2 获取和设置ValueBinding对象的属性
45 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：
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47 public abstract Object getValue(FacesContext facesContext)
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49 throws javax.faces.el.PropertyNotFoundException
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51 比如，下面的代码提取由一个ValueBinding表示的ShoppingCartBean对象：
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53 FacesContext facesContext = FacesContext.getCurrentInstance();
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55 ApplicationFactory factory = (ApplicationFactory)
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57 FactoryFinder.getFactory(FactoryFinder.APPLICATION_FACTORY);
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59 Application application = factory.getApplication();
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61 ValueBinding valueBinding =
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63 application.getValueBinding("shoppingCartBean");
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65 ShoppingCartBean bean =
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67 (ShoppingCartBean) valueBinding.getValue(facesContext);
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69 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：
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71 public abstract void setValue
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73 (FacesContext facesContext, Object value)
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75 throws javax.faces.el.PropertyNotFoundException
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77 比如，假设ShoppingCartBean有一个叫作purchaseId的属性，其声明如下：
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79 private String purchaseId;
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81 public String getPurchaseId() {
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83 return purchaseId;
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85 }
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87 public void setPurchaseId(String purchaseId) {
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89 this.purchaseId = purchaseId;
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91 }
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93 则下面的代码可获取ShoppingCartBean对象的purchaseId属性，并将其值设为12345。
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95 FacesContext facesContext = FacesContext.getCurrentInstance();
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97 ApplicationFactory factory = (ApplicationFactory)
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99 FactoryFinder.getFactory(FactoryFinder.APPLICATION_FACTORY);
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101 Application application = factory.getApplication();
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103 ValueBinding valueBinding =
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105 application.getValueBinding("ShoppingCartBean.purchaseId");
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107 valueBinding.setValue(facesContext, "12345");
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109 // print the current value
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111 System.out.println("Purchase Id:" +
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113 valueBinding.getValue(facesContext));
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115 这段代码会在控制台输出如下文本：
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117 Purchase Id: 12345
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119 3.6.3 检查ValueBinding属性是否可写
120 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：
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122 public abstract boolean isReadOnly(FacesContext facesContext)
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124 throws javax.faces.el.PropertyNotFoundException
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126 比如，下面的代码在改变ShoppingCartBean的purchaseId属性前先检查其是否可写。
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128 FacesContext facesContext = FacesContext.getCurrentInstance();
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130 ApplicationFactory factory = (ApplicationFactory)
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132 FactoryFinder.getFactory(FactoryFinder.APPLICATION_FACTORY);
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134 Application application = factory.getApplication();
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136 ValueBinding valueBinding =
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138 application.getValueBinding("shoppingCartBean.purchaseId");
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140 if (!valueBinding.isReadOnly(facesContext))
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142 valueBinding.setValue(facesContext, "12345");
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