如何解决此反射错误?java.lang.reflect.Field.get (Field.java: 393)

Question

我试图在运行时更改@ColumnTransformer的值，以便我可以将我的加密密钥存储在服务器上的其他地方。我该怎么做呢?

下面是我的主代码:

List<Field> fields = Arrays.asList(User.class.getDeclaredFields());
for(Field field:fields)
{
if(field.toString().endsWith("secretfield"))
{
System.out.println(field);
List<Annotation> annotations = Arrays.asList(field.getDeclaredAnnotations());
for(Annotation annotation : annotations)
{
if(annotation.annotationType().toString().endsWith("ColumnTransformer"))
{
Annotation newAnnotation = new ColumnTransformer(){
@Override
public String forColumn() {
return "secretfield";
}
@Override
public String read(){
return "This is Sparta";
}
@Override
public String write(){
return "This is also sparta";
}
@Override
public Class<? extends Annotation> annotationType() {
return annotation.annotationType();
}
};
Field field1 = User.class.getDeclaredField("secretfield");
field1.setAccessible(true);
Map<Class<? extends Annotation>, Annotation> annotations1 = (Map<Class<? extends Annotation>, Annotation>) field1.get(User.class);
annotations1.put(ColumnTransformer.class, newAnnotation);
}
}
}
}

下面是我的Pojo:

package Model;
import org.hibernate.annotations.ColumnTransformer;

import javax.persistence.*;
import javax.sql.rowset.serial.SerialBlob;
import java.sql.Blob;
import java.io.Serializable;
@Entity(name = "user")
@Table(name = "user")
public class User implements Serializable {
@Column(name = "secretfield", nullable = true)
@ColumnTransformer(read = "TEST")
private String secretfield;
public String getSecretfield() {
return secretfield;
}
public void setSecretfield(String secretfield) {
this.secretfield = secretfield;
}
}

我得到这个错误:

Exception in thread "main" java.lang.IllegalArgumentException: Can not set java.lang.String field Model.User.secretfield to null value
at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:167)
at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:171)
at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:58)
at sun.reflect.UnsafeObjectFieldAccessorImpl.get(UnsafeObjectFieldAccessorImpl.java:36)
at java.lang.reflect.Field.get(Field.java:393)
at Runner.main(Runner.java:65)

Answer 1

这一行

Map<Class<? extends Annotation>, Annotation> annotations1 = (Map<Class<? extends Annotation>, Annotation>) field1.get(User.class);

是不正确的。field1.get正在检索字段值，而不是注释映射。

由于您只需要修改注释中的参数，因此实际上不需要创建ColumnTransformer的新实例。请参考在运行时修改类定义的注释字符串参数了解详细信息。

changeAnnotationValue稍作修改，修复ColumnTransformer缺少默认值导致的NPE，如下图所示。

import org.hibernate.annotations.ColumnTransformer;
import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Proxy;
import java.util.Map;
public class FieldGetAnnotation {
public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {
Field field = User.class.getDeclaredField("secretfield");
field.setAccessible(true);
final ColumnTransformer fieldAnnotation = field.getAnnotation(ColumnTransformer.class);
System.out.println("old FieldAnnotation = " + fieldAnnotation.forColumn());
changeAnnotationValue(fieldAnnotation, "forColumn", "secretfield");
System.out.println("modified FieldAnnotation = " + fieldAnnotation.forColumn());
System.out.println("old FieldAnnotation = " + fieldAnnotation.read());
changeAnnotationValue(fieldAnnotation, "read", "This is Sparta");
System.out.println("modified FieldAnnotation = " + fieldAnnotation.read());
System.out.println("old FieldAnnotation = " + fieldAnnotation.write());
changeAnnotationValue(fieldAnnotation, "write", "This is also sparta");
System.out.println("modified FieldAnnotation = " + fieldAnnotation.write());
}
@SuppressWarnings("unchecked")
public static Object changeAnnotationValue(Annotation annotation, String key, Object newValue) {
Object handler = Proxy.getInvocationHandler(annotation);
Field f;
try {
f = handler.getClass().getDeclaredField("memberValues");
} catch (NoSuchFieldException | SecurityException e) {
throw new IllegalStateException(e);
}
f.setAccessible(true);
Map<String, Object> memberValues;
try {
memberValues = (Map<String, Object>) f.get(handler);
} catch (IllegalArgumentException | IllegalAccessException e) {
throw new IllegalStateException(e);
}
Object oldValue = memberValues.get(key);
if (oldValue != null && oldValue.getClass() != newValue.getClass()) {
throw new IllegalArgumentException();
}
memberValues.put(key, newValue);
return oldValue;
}
}