使用自动生成的Classendpoint插入方法插入实体时抛出Nullpointerexception

Question

我很困惑使用自动生成的端点类.我想使用生成的端点将新对象插入数据存储区.但是,抛出一个例外.

fooEndpoint.insertFoo(foo); // throws null pointer exception 

我的实体类与此源中的给定示例类似:https: //developers.google.com/appengine/docs/java/datastore/jpa/overview.

这是我的实体:

@Entity
public class Foo {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Key ID;

这是堆栈跟踪:

java.lang.NullPointerException
at org.datanucleus.api.jpa.JPAEntityManager.find(JPAEntityManager.java:318)
at org.datanucleus.api.jpa.JPAEntityManager.find(JPAEntityManager.java:256)
at com.FooEndpoint.containsFoo(FooEndpoint.java:150)
at com.FooEndpoint.insertFoo(FooEndpoint.java:96)

另一方面,当我使用EntityManager persist方法时,我可以插入新对象.因为,这不会检查数据存储上是否存在.

我希望,classEndpoint插入方法应该保存对象并将自动键分配给ID字段.

或者我需要初始化ID字段.

这是自动生成的端点类insertFoo方法.

  /**
 * This inserts a new entity into App Engine datastore. If the entity already
 * exists in the datastore, an exception is thrown.
 * It uses HTTP POST method.
 *
 * @param foo the entity to be inserted.
 * @return The inserted entity.
 */
public Foo insertFoo(Foo foo) {
    EntityManager mgr = getEntityManager();
    try {
        if (containsFoo(foo)) {
            throw new EntityExistsException("Object already exists");
        }
        mgr.persist(foo);
    } finally {
        mgr.close();
    }
    return foo;
}

这是containsFoo方法

    private boolean containsFoo(Foo foo) {
    EntityManager mgr = getEntityManager();
    boolean contains = true;
    try {
        Foo item = mgr.find(Foo.class, foo.getID());  // exception occurs here
        if (item == null) {
            contains = false;
        }
    } finally {
        mgr.close();
    }
    return contains;
}

foo.getID()为null.因为,它是新的对象.我期待着,app引擎为它创建了一个密钥.或者我需要为它明确创建一个密钥？

Foo类中的其他字段是简单类型,例如String和booleans.

谢谢你的时间.

Answer 1

我有完全相同的问题.我将介绍我的工作方式.

原始自动生成的Endpoints类相关代码:

private boolean containsFoo(Foo foo) {
    EntityManager mgr = getEntityManager(); 
    boolean contains = true;
    try {
        Foo item = mgr.find(Foo.class, foo.getID());
        if (item == null) {
            contains = false;
        }
    } finally {
        mgr.close();
    }
    return contains;
}

更改了相关代码,以包含对作为参数传递的实体对象的空检查.

private boolean containsFoo(Foo foo) {
    EntityManager mgr = getEntityManager(); 
    boolean contains = true;
    try {
        // If no ID was set, the entity doesn't exist yet.
        if(foo.getID() == null)
            return false;
        Foo item = mgr.find(Foo.class, foo.getID());
        if (item == null) {
            contains = false;
        }
    } finally {
        mgr.close();
    }
    return contains;
}

通过这种方式,它可以正常运行,但我相信会出现更有经验的答案和解释.