Troubleshooting

这是开发人员遇到的一些常见问题，并解释了为什么会发生以及如何解决这些问题。

IllegalArgumentException 由于类型不可转换

FieldTrouble示例将生成IllegalArgumentException。调用Field.setInt()以设置具有原始类型值的参考类型Integer的字段。在非反射等效项Integer val = 42中，编译器会将原始类型42转换(或* box *)为参考类型new Integer(42)，以便其类型检查将接受该语句。使用反射时，类型检查仅在运行时发生，因此没有机会将值装箱。

import java.lang.reflect.Field;

public class FieldTrouble {
    public Integer val;

    public static void main(String... args) {
	FieldTrouble ft = new FieldTrouble();
	try {
	    Class<?> c = ft.getClass();
	    Field f = c.getDeclaredField("val");
  	    f.setInt(ft, 42);               // IllegalArgumentException

        // production code should handle these exceptions more gracefully
	} catch (NoSuchFieldException x) {
	    x.printStackTrace();
 	} catch (IllegalAccessException x) {
 	    x.printStackTrace();
	}
    }
}

$ java FieldTrouble
Exception in thread "main" java.lang.IllegalArgumentException: Can not set
  java.lang.Object field FieldTrouble.val to (long)42
        at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException
          (UnsafeFieldAccessorImpl.java:146)
        at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException
          (UnsafeFieldAccessorImpl.java:174)
        at sun.reflect.UnsafeObjectFieldAccessorImpl.setLong
          (UnsafeObjectFieldAccessorImpl.java:102)
        at java.lang.reflect.Field.setLong(Field.java:831)
        at FieldTrouble.main(FieldTrouble.java:11)

为了消除此异常，应将以下行Field.set(Object obj，Object value)替换为有问题的行：

f.set(ft, new Integer(43));

Integer.class.isAssignableFrom(int.class) == false

int.class.isAssignableFrom(Integer.class) == false

非公共字段的 NoSuchFieldException

精明的 Reader 可能会注意到，如果使用前面显示的FieldSpy示例来获取有关非公共字段的信息，它将失败：

$ java FieldSpy java.lang.String count
java.lang.NoSuchFieldException: count
        at java.lang.Class.getField(Class.java:1519)
        at FieldSpy.main(FieldSpy.java:12)

IllegalAccessException 修改final字段时

如果try获取或设置private或其他不可访问字段的值或设置final字段的值(无论其访问修饰符如何)，都可能引发IllegalAccessException。

FieldTroubleToo示例说明了try设置final字段而导致的堆栈跟踪的类型。

import java.lang.reflect.Field;

public class FieldTroubleToo {
    public final boolean b = true;

    public static void main(String... args) {
	FieldTroubleToo ft = new FieldTroubleToo();
	try {
	    Class<?> c = ft.getClass();
	    Field f = c.getDeclaredField("b");
// 	    f.setAccessible(true);  // solution
	    f.setBoolean(ft, Boolean.FALSE);   // IllegalAccessException

        // production code should handle these exceptions more gracefully
	} catch (NoSuchFieldException x) {
	    x.printStackTrace();
	} catch (IllegalArgumentException x) {
	    x.printStackTrace();
	} catch (IllegalAccessException x) {
	    x.printStackTrace();
	}
    }
}

$ java FieldTroubleToo
java.lang.IllegalAccessException: Can not set final boolean field
  FieldTroubleToo.b to (boolean)false
        at sun.reflect.UnsafeFieldAccessorImpl.
          throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:55)
        at sun.reflect.UnsafeFieldAccessorImpl.
          throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:63)
        at sun.reflect.UnsafeQualifiedBooleanFieldAccessorImpl.setBoolean
          (UnsafeQualifiedBooleanFieldAccessorImpl.java:78)
        at java.lang.reflect.Field.setBoolean(Field.java:686)
        at FieldTroubleToo.main(FieldTroubleToo.java:12)