来自另一个托管bean的托管bean的访问权限属性 [英] Cant access property of managed bean from another managed bean
问题描述
我想使用@ManagedProperty
访问另一个bean中的@SessionScoped
bean的属性.简而言之,我想在secondBean中访问firstBean的name属性.
I want to access the property of a @SessionScoped
bean in another bean using @ManagedProperty
. In short, I want to access the name property of firstBean in secondBean.
@ManagedBean
@SessionScoped
public class FirstBean implements Serializable{
private String name;
//...other attributes
//...constructor
public String getSelectedModel() {
return selectedModel;
}
public void setSelectedModel(String selectedModel) {
this.selectedModel = selectedModel;
}
//other getters&setters
}
第二个bean:
@ManagedBean
@SessionScoped
public class SecondBean implements Serializable{
@ManagedProperty(value="#{firstBean}")
private FirstBean firstBean
public SecondBean() {
System.out.println(firstBean.getName());
}
public IndexBean getFirstBean() {
return firstBean;
}
public void setFirstBean(FirstBean firstBean) {
this.firstBean = firstBean;
}
运行此命令时,总是在第二个bean的构造函数中的System.out.println(firstBean.getName());
上获得NullPointerException
,这似乎意味着我需要创建firstBean
的新实例.
When I run this, I always get NullPointerException
on System.out.println(firstBean.getName());
in the constructor of second bean, which seems to mean that I need to create a new instance of firstBean
.
但是奇怪的是,当我注释掉这一行时,我可以做这样的事情而没有错误,这意味着firstBean实际上是secondBean的属性.
But strangely, when I commented out this line, I can do something like this with no errors, which means that firstBean is actually a property of secondBean.
<h:outputText value="#{secondBean.firstBean.name}" />
这是什么问题?
推荐答案
无法在构造函数中访问注入的依赖项.您基本上希望Java能够执行以下操作:
It's not possible to access an injected dependency in the constructor. You're basically expecting that Java is able to do something like this:
SecondBean secondBean; // Declare.
secondBean.firstBean = new FirstBean(); // Inject.
secondBean = new SecondBean(); // Construct.
如果尚未构造实例,则绝对不可能设置实例变量.相反,它的工作方式如下:
It's absolutely not possible to set an instance variable if the instance is not constructed yet. Instead, it works as follows:
SecondBean secondBean; // Declare.
secondBean = new SecondBean(); // Construct.
secondBean.firstBean = new FirstBean(); // Inject.
然后,为了基于注入的依赖关系执行业务操作,请使用带有@PostConstruct
注释的方法.构造和依赖项注入后,依赖关系注入管理器将直接调用它.
Then, in order to perform business actions based on injected dependencies, use a method annotated with @PostConstruct
. It will be invoked by the dependency injection manager directly after construction and dependency injection.
所以,只需替换
public SecondBean() {
System.out.println(firstBean.getName());
}
作者
@PostConstruct
public void init() { // Note: method name is fully to your choice.
System.out.println(firstBean.getName());
}
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