Maven：项目无法产生Maven Dependencies且无法update project报cannot nest错误

找自己出错项目的.classpath文件，打开，ctrl+f，找是否有以下文件，如果没有在末尾加进去，再refresh项目即可：

<classpathentry kind="con" path="org.eclipse.m2e.MAVEN2_CLASSPATH_CONTAINER">
    <attributes>
        <attribute name="maven.pomderived" value="true"/>
        <attribute name="org.eclipse.jst.component.dependency" value="/WEB-INF/lib"/>
    </attributes>
</classpathentry>

添加后我的.classpath文件是这样的：

<?xml version="1.0" encoding="UTF-8"?>
<classpath>
    <classpathentry kind="src" path="src/main/java"/>
    <classpathentry kind="src" path="src/main/resources"/>
    <classpathentry kind="src" path="src/extend/java"/>
    <classpathentry kind="con" path="org.maven.ide.eclipse.MAVEN2_CLASSPATH_CONTAINER">
    <attributes>
        <attribute name="org.eclipse.jst.component.dependency" value="/WEB-INF/lib"/>
    </attributes>
    </classpathentry>
    <classpathentry kind="con" path="org.eclipse.jst.j2ee.internal.web.container"/>
    <classpathentry kind="con" path="org.eclipse.jst.server.core.container/org.eclipse.jst.server.tomcat.runtimeTarget/Apache Tomcat v7.0 (2)">
    <attributes>
        <attribute name="owner.project.facets" value="jst.web"/>
    </attributes>
    </classpathentry>
    <classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/jre7">
    <attributes>
        <attribute name="owner.project.facets" value="java"/>
    </attributes>
    </classpathentry>
    <classpathentry kind="con" path="org.eclipse.m2e.MAVEN2_CLASSPATH_CONTAINER">
    <attributes>
        <attribute name="maven.pomderived" value="true"/>
        <attribute name="org.eclipse.jst.component.dependency" value="/WEB-INF/lib"/>
    </attributes>
    </classpathentry>
    <classpathentry kind="output" path="build/classes"/>
</classpath>

这样项目就没有什么问题了！

项目加载是要读取.classpath文件的，加载不到且Maven不能update project说cannot nest错误。