为什么 grails 第一次访问 hasMany 关系时会抛出空指针异常？
我有一个奇怪的问题。
我有两个域类 User 和 Post ，其中包含以下字段：
class User {
String name
static hasMany = [posts: Post]
static constraints = { }
}
和
class Post {
String content
long date = System.getTimeInMillis()
static constraints = { }
static belongsTo = [user: User]
static mapping = {
version: 'false'
}
}
控制器代码是：
class UserController {
def addUser = {
def user
if (User.count() == 0) {
user = new User()
user.name = "Manish Zedwal"
user.save(flush: true)
} else {
user = User.get(1)
}
println "Posts count: " + user.posts.size()
render "post count: " + user.posts.size()
}
}
第一次访问 url http://localhost:8080/test /user/addUser，它抛出空指针异常，但此后工作正常。
遇到的例外
2011-08-04 15:41:25,847 [http-8080-1] ERROR errors.GrailsExceptionResolver - Exception occurred when processing request: [GET] /test/user/addUser
Stacktrace follows:
java.lang.NullPointerException: Cannot invoke method size() on null object
at test.UserController$_closure2.doCall(UserController.groovy:18)
at test.UserController$_closure2.doCall(UserController.groovy)
at java.lang.Thread.run(Thread.java:636)
这是我第二次
Posts count: 0
，它像魅力一样在用户域类中打印和渲染得很好，因为 hasMany 关系为 posts，posts< /code> 是 Post 对象的列表，那么在获取空列表的大小时不应该出现空指针异常，而应该为零。
任何帮助表示赞赏
I have a strange problem.
I have two domain classes User and Post with fields:
class User {
String name
static hasMany = [posts: Post]
static constraints = { }
}
and
class Post {
String content
long date = System.getTimeInMillis()
static constraints = { }
static belongsTo = [user: User]
static mapping = {
version: 'false'
}
}
and controller code is:
class UserController {
def addUser = {
def user
if (User.count() == 0) {
user = new User()
user.name = "Manish Zedwal"
user.save(flush: true)
} else {
user = User.get(1)
}
println "Posts count: " + user.posts.size()
render "post count: " + user.posts.size()
}
}
For the first time while accessing url http://localhost:8080/test/user/addUser, it throws null pointer exception, but after this works fine.
This is the exception I am getting
2011-08-04 15:41:25,847 [http-8080-1] ERROR errors.GrailsExceptionResolver - Exception occurred when processing request: [GET] /test/user/addUser
Stacktrace follows:
java.lang.NullPointerException: Cannot invoke method size() on null object
at test.UserController$_closure2.doCall(UserController.groovy:18)
at test.UserController$_closure2.doCall(UserController.groovy)
at java.lang.Thread.run(Thread.java:636)
and for second time, it prints and renders fine like charm
Posts count: 0
In user domain class, coz of hasMany relationship for posts, posts is a list of Post objects then there shouldn't be null pointer exception on getting the size of empty list, rather it should be zero.
Any help appreciated
如果你对这篇内容有疑问，欢迎到本站社区发帖提问 参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。
绑定邮箱获取回复消息
由于您还没有绑定你的真实邮箱，如果其他用户或者作者回复了您的评论，将不能在第一时间通知您！
发布评论
评论（2）
当您映射这样的集合时，
hasMany声明会向您的集合添加一个具有指定名称（在本例中为posts）的Set类型字段。班级。但它不会初始化该集合，因此它最初为空。当您调用addToPosts时，它会检查它是否为 null，并在必要时创建一个新的空 Set，并将 Post 添加到集合中。但如果您不调用addToPosts或显式初始化该集合，它将为 null。当您从数据库加载
User时，Hibernate将填充所有字段，并且集合包含在其中。它创建一个空的新 Set（一个修改感知PersistentSet），并向其中添加实例（如果有）。不过，调用save()不会从数据库重新加载实例，因此 null 集仍然为 null。为了让类在新类和持久类时都以相同的方式运行，您可以向类中添加一个字段，就像 Rob 在他的答案中所示，初始化为一个空集 (
Set posts = []）When you map a collection like that, the
hasManydeclaration adds a field of typeSetwith the specified name (in this caseposts) to your class. It doesn't initialize the set though, so it's initially null. When you calladdToPostsit checks if it's null and creates a new empty Set if necessary, and adds the Post to the collection. But if you don't calladdToPostsor explicitly initialize the set, it will be null.When you load a
Userfrom the database, Hibernate will populate all the fields, and the collection is included in that. It creates a new Set (a modification-awarePersistentSet) that's empty, and adds instances to it if there are any. Callingsave()doesn't reload the instance from the database though, so the null set will still be null.To get the class to behave the same way when it's new and when it's persistent, you can add a field to your class Like Rob showed in his answer, initialized to an empty set (
Set posts = [])您可以通过在映射旁边显式声明集合属性（带有值）来防止这种情况：
您可以定义所需的集合类型。默认为
Set，但如果您需要保持顺序，您可以考虑List或SortedSet。You can prevent this by explicitly declaring your collection property (with a value) alongside your mapping:
You can define the collection type you need. The default is
Set, but if you need to maintain order, you might considerListorSortedSet.