Question

我正试图通过Class clazz实例化一个新对象：

代码：

public class EntityType {
    private Map json = null;
    public EntityType(Map json){
        this.json = json;
    }
    public <T> T getAs(Class<T> clazz){
        final Class<T> clz = clazz;
        T obj = null;
        try {
            if (checkMap(json)){
                obj = clz.newInstance();
            }
        } catch (Exception e){
            if (e instanceof InstantiationException){
                e.printStackTrace();
            } else if (e instanceof IllegalAccessException){
                e.printStackTrace();
            } else {
                e.printStackTrace();
            }
        }
        return obj;
    }
}

这就是我使用代码的方式：

EntityType type = new EntityType(map); // map contains values 
User user = type.getAs(User.class);

上述代码抛出

java.lang.InstantiationException。

我的代码可能有什么问题？

更新

    @Entity(name = "user")
    public class User { 
        @Id 
        private String id;
        @Column
        private String name;
        @Column
        private Long age;
        public User() {}
       // Setters and getter 
   }

完整堆栈跟踪：

java.lang.InstantiationException: org.goo.AnnotationTest$User
    at java.lang.Class.newInstance0(Class.java:357)
    at java.lang.Class.newInstance(Class.java:325)
    at org.goodb.client.Goo$EntityType.getAs(Goo.java:90)
    at org.goo.AnnotationTest.doTest(AnnotationTest.java:97)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
    at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:74)
    at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:31)
    at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:83)
    at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:72)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:231)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
    at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
    at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:71)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:174)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

Answer 1

您的用户类中可能没有默认构造函数

Answer 2

异常堆栈跟踪说

java.lang.InstantiationException: org.goo.AnnotationTest$User

表明User是嵌套或内部类而不是顶级类。静态嵌套类的行为类似于顶级类，但对于非静态内部类，每个实例都与其包含类的特定实例相关联，并且编译器在参数列表的前面添加一个额外参数，以便每个构造函数都传入适当的包含实例。源代码中的内容是什么

public User() {}

实际上在字节码中对应于

public User(AnnotationTest containingInstance) { this.container = containingInstance; }

（不是那些参数/字段名称，但你明白了）。所以你的User类实际上没有无参构造函数。

如果您将User类声明为顶级类或将其标记为static，那么它将按预期工作。

Answer 3

您有一个通用方法，但您没有传递类型参数。试试这个：

 User user = type.<User>getAs(User.class);

如果您与我们分享更多代码，我可以为您提供更多帮助。例如，我想看到完整的堆栈跟踪和User类。

Answer 4

根据javadoc描述：

 * @exception  InstantiationException
 *               if this {@code Class} represents an abstract class,
 *               an interface, an array class, a primitive type, or void;
 *               or if the class has no nullary constructor;
 *               or if the instantiation fails for some other reason.

我认为你没有“nullary constructor”，或者很可能说：默认构造函数。

实例化泛型类型

4 个答案: