NullPoInterException，但是为什么？

Question

我上课的地方

    @Enumerated(EnumType.STRING)
private StateOfDispatchToWD stateOfDispatchToWD = StateOfDispatchToWD.NONE;

，getter和setter：

public StateOfDispatchToWD getStateOfDispatchToWD() {
    return stateOfDispatchToWD;
}

public void setStateOfDispatchToWD(StateOfDispatchToWD stateOfDispatchToWD) {
    this.stateOfDispatchToWD = stateOfDispatchToWD;
}

和方法：

public boolean isSuccessfullySentToWD() {
    return zoj.getStateOfDispatchToWD() == StateOfDispatchToWD.SUCCESSFULLY_DISPATCH;
}

从JSF打电话：

            <p:outputLabel value="OK"
                           rendered="#{Z.isSuccessfullySentToWD()}"/>

有时我得到：

[javax.enterprise.resource.webcontainer.jsf.lifecycle]
javax.el.ELException: //content/resources/zo.xhtml @240,92 rendered="#{Z.isSuccessfullySentToWD()}":
java.lang.NullPointerException:
javax.faces.FacesException:
javax.el.ELException: //content/resources/zo.xhtml @240,92 rendered="#{Z.isSuccessfullySentToWD()}":
java.lang.NullPointerException

但是为什么呢？恐怕我不明白。多谢。

Answer 1

Z的范围可能是错误的，请尝试将其放入会话或应用程序范围