Java反射getFields方法和getDeclaredFields方法的区别

最近在写注解相关代码的时候经常会使用到反射机制，关于Class类的两个方法getFields和getDeclaredFields有一些疑惑，这两个方法到底哪里不一样，有什么区别呢。最好的教程就是源码，直接进入源码可以看到如下注释：

/**
     * Returns an array containing {@code Field} objects reflecting all
     * the accessible public fields of the class or interface represented by
     * this {@code Class} object.
     *
     * 
 If this {@code Class} object represents a class or interface with no
     * no accessible public fields, then this method returns an array of length
     * 0.
     *
     * 
 If this {@code Class} object represents a class, then this method
     * returns the public fields of the class and of all its superclasses.
     *
     * 
 If this {@code Class} object represents an interface, then this
     * method returns the fields of the interface and of all its
     * superinterfaces.
     *
     * 
 If this {@code Class} object represents an array type, a primitive
     * type, or void, then this method returns an array of length 0.
     *
     * 
 The elements in the returned array are not sorted and are not in any
     * particular order.
     *
     * @return the array of {@code Field} objects representing the
     *         public fields
     * @throws SecurityException
     *         If a security manager, s, is present and
     *         the caller's class loader is not the same as or an
     *         ancestor of the class loader for the current class and
     *         invocation of {@link SecurityManager#checkPackageAccess
     *         s.checkPackageAccess()} denies access to the package
     *         of this class.
     *
     * @since JDK1.1
     * @jls 8.2 Class Members
     * @jls 8.3 Field Declarations
     */
    @CallerSensitive
    public Field[] getFields() throws SecurityException {
        checkMemberAccess(Member.PUBLIC, Reflection.getCallerClass(), true);
        return copyFields(privateGetPublicFields(null));
    }

/**
     * Returns an array of {@code Field} objects reflecting all the fields
     * declared by the class or interface represented by this
     * {@code Class} object. This includes public, protected, default
     * (package) access, and private fields, but excludes inherited fields.
     *
     * 
 If this {@code Class} object represents a class or interface with no
     * declared fields, then this method returns an array of length 0.
     *
     * 
 If this {@code Class} object represents an array type, a primitive
     * type, or void, then this method returns an array of length 0.
     *
     * 
 The elements in the returned array are not sorted and are not in any
     * particular order.
     *
     * @return  the array of {@code Field} objects representing all the
     *          declared fields of this class
     * @throws  SecurityException
     *          If a security manager, s, is present and any of the
     *          following conditions is met:
     *
     *          

     *
     *          
 the caller's class loader is not the same as the
     *          class loader of this class and invocation of
     *          {@link SecurityManager#checkPermission
     *          s.checkPermission} method with
     *          {@code RuntimePermission("accessDeclaredMembers")}
     *          denies access to the declared fields within this class
     *
     *          
 the caller's class loader is not the same as or an
     *          ancestor of the class loader for the current class and
     *          invocation of {@link SecurityManager#checkPackageAccess
     *          s.checkPackageAccess()} denies access to the package
     *          of this class
     *
     *          
     *
     * @since JDK1.1
     * @jls 8.2 Class Members
     * @jls 8.3 Field Declarations
     */
    @CallerSensitive
    public Field[] getDeclaredFields() throws SecurityException {
        checkMemberAccess(Member.DECLARED, Reflection.getCallerClass(), true);
        return copyFields(privateGetDeclaredFields(false));
    }

以上注释的意思大概就是说：

• getFields方法只会返回public修饰符修饰的属性，且能返回父类属性
• getDeclaredFields方法会返回当前类的所有声明属性，包括public、private、protected、default、static修饰符修饰的属性，但是不能返回父类的属性。

---

下面，以一个例子说明，更加简洁易懂：

/**
 * 父类
 */
public class OldStrudent {
    private String oldName;
    public boolean oldSex;
}

/**
 * 子类
 */
public class Student extends OldStrudent {
    private String name;
    private Integer score;
    private boolean sex;
    public String teacher;
    private static String id;
}

/**
 * 测试类
 */
import java.lang.reflect.Field;

public class Main {

    public static void main(String[] args) {
        Field[] fields = Student.class.getFields();
        for (Field field : fields) {
            System.out.println(field.getName());
        }

        System.out.println("---------");
        Field[] decFields = Student.class.getDeclaredFields();
        for (Field field : decFields) {
            System.out.println(field.getName());
        }
    }
}
/**
 * 输出结果：
 * teacher
 * oldSex
 * ---------
 * name
 * score
 * sex
 * teacher
 * id
 */

同理，Class的getMethod和getDeclaredMethods方法也是这个逻辑。