java基础源码 (5)--reflect包-AccessibleObject类

学习参考博客：https://blog.csdn.net/benjaminzhang666/article/details/9664585
AccessibleObject类基本作用
　　1。将反射的对象标记为在使用时取消默认java语言访问控制检查的能力
　　2。在反射对象中设置accessible（翻译：无障碍）标志允许具有足够的特权

/**
 * The AccessibleObject class is the base class for Field, Method and
 * Constructor objects.  It provides the ability to flag a reflected
 * object as suppressing default Java language access control checks
 * when it is used.  The access checks--for public, default (package)
 * access, protected, and private members--are performed when Fields,
 * Methods or Constructors are used to set or get fields, to invoke
 * methods, or to create and initialize new instances of classes,
 * respectively.
　　AccessibleObject类是Field,Method和Constructor对象的父类，它提供了将反射对象标记
为在使用它时抑制默认java语言访问控制检查的功能。当使用Fields，Methods或Constructor来
设置或获取字段，调用方法，或创建和初始化新的类实例时，执行访问检查（对于public,默认（包）访问
，受保护和私有成员）
 *
 * <p>Setting the {@code accessible} flag in a reflected object
 * permits sophisticated applications with sufficient privilege, such
 * as Java Object Serialization or other persistence mechanisms, to
 * manipulate objects in a manner that would normally be prohibited.
 在反射对象中设置accessible标志允许具有足够权限的复杂应用程序（如java对象序列化
或其他持久性机制）以通常被禁止的方式操纵对象
 * <p>By default, a reflected object is <em>not</em> accessible.
　　默认情况下，反射对象不可访问
*/
public class AccessibleObject implements AnnotatedElement {

    /**
     * The Permission object that is used to check whether a client
     * has sufficient privilege to defeat Java language access
     * control checks.
　　　　Permission对象，用于检查客户端是否具有足够的权限来阻止java语言访问控制检查
     */
    static final private java.security.Permission ACCESS_PERMISSION =
        new ReflectPermission("suppressAccessChecks");

    /**
     * Convenience method to set the {@code accessible} flag for an
     * array of objects with a single security check (for efficiency).
　　　　使用单一安全检查来设置对象数组的可访问标志的一个方便的方法（为了效率）*/
    public static void setAccessible(AccessibleObject[] array, boolean flag)
        throws SecurityException {
        SecurityManager sm = System.getSecurityManager();//获取系统安全的接口
        if (sm != null) sm.checkPermission(ACCESS_PERMISSION);
        for (int i = 0; i < array.length; i++) {
            setAccessible0(array[i], flag);
        }
    }

    /**
  　　将对象的可访问标志设置为指示的布尔值*/
    public void setAccessible(boolean flag) throws SecurityException {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) sm.checkPermission(ACCESS_PERMISSION);
        setAccessible0(this, flag);
    }

    /* Check that you aren't exposing java.lang.Class.<init> or sensitive
       fields in java.lang.Class. */
    private static void setAccessible0(AccessibleObject obj, boolean flag)
        throws SecurityException
    {
        if (obj instanceof Constructor && flag == true) {
            Constructor<?> c = (Constructor<?>)obj;
            if (c.getDeclaringClass() == Class.class) {
                throw new SecurityException("Cannot make a java.lang.Class" +
                                            " constructor accessible");
            }
        }
        obj.override = flag;
    }

    /**
　　　获取此对象的accessible标志的值（布尔类型）*/
    public boolean isAccessible() {
        return override;
    }

    /**
     * Constructor: only used by the Java Virtual Machine.
　　　　构造函数，仅有java虚拟机使用
     */
    protected AccessibleObject() {}

    // Indicates whether language-level access checks are overridden
    // by this object. Initializes to "false". This field is used by
    // Field, Method, and Constructor.
    通过这个对象，指示是否覆盖语言级别访问检查权限，初始化为false,该字段用于Field,
　　Method和Constructor
    // NOTE: for security purposes, this field must not be visible
    // outside this package.
　　出于安全考虑，此字段不得显示出包外
    boolean override;

    // Reflection factory used by subclasses for creating field,
    // method, and constructor accessors. Note that this is called
    // very early in the bootstrapping process.
　　　　子类用于创建字段，方法和构造函数的反射工厂
    static final ReflectionFactory reflectionFactory =
        AccessController.doPrivileged(
            new sun.reflect.ReflectionFactory.GetReflectionFactoryAction());

    /**
     * @throws NullPointerException {@inheritDoc}
     * @since 1.5
     */
    public <T extends Annotation> T getAnnotation(Class<T> annotationClass) {
        throw new AssertionError("All subclasses should override this method");
    }

    /**
     * {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     * @since 1.5
     */
    @Override
    public boolean isAnnotationPresent(Class<? extends Annotation> annotationClass) {
        return AnnotatedElement.super.isAnnotationPresent(annotationClass);
    }

   /**
     * @throws NullPointerException {@inheritDoc}
     * @since 1.8
     */
    @Override
    public <T extends Annotation> T[] getAnnotationsByType(Class<T> annotationClass) {
        throw new AssertionError("All subclasses should override this method");
    }

    /**
     * @since 1.5
     */
    public Annotation[] getAnnotations() {
        return getDeclaredAnnotations();
    }

    /**
     * @throws NullPointerException {@inheritDoc}
     * @since 1.8
     */
    @Override
    public <T extends Annotation> T getDeclaredAnnotation(Class<T> annotationClass) {
        // Only annotations on classes are inherited, for all other
        // objects getDeclaredAnnotation is the same as
        // getAnnotation.
        return getAnnotation(annotationClass);
    }

    /**
     * @throws NullPointerException {@inheritDoc}
     * @since 1.8
     */
    @Override
    public <T extends Annotation> T[] getDeclaredAnnotationsByType(Class<T> annotationClass) {
        // Only annotations on classes are inherited, for all other
        // objects getDeclaredAnnotationsByType is the same as
        // getAnnotationsByType.
        return getAnnotationsByType(annotationClass);
    }

    /**
     * @since 1.5
     */
    public Annotation[] getDeclaredAnnotations()  {
        throw new AssertionError("All subclasses should override this method");
    }

    // Shared access checking logic.

    // For non-public members or members in package-private classes,
    // it is necessary to perform somewhat expensive security checks.
    // If the security check succeeds for a given class, it will
    // always succeed (it is not affected by the granting or revoking
    // of permissions); we speed up the check in the common case by
    // remembering the last Class for which the check succeeded.
    //
    // The simple security check for Constructor is to see if
    // the caller has already been seen, verified, and cached.
    // (See also Class.newInstance(), which uses a similar method.)
    //
    // A more complicated security check cache is needed for Method and Field
    // The cache can be either null (empty cache), a 2-array of {caller,target},
    // or a caller (with target implicitly equal to this.clazz).
    // In the 2-array case, the target is always different from the clazz.
    volatile Object securityCheckCache;

    void checkAccess(Class<?> caller, Class<?> clazz, Object obj, int modifiers)
        throws IllegalAccessException
    {
        if (caller == clazz) {  // quick check
            return;             // ACCESS IS OK
        }
        Object cache = securityCheckCache;  // read volatile
        Class<?> targetClass = clazz;
        if (obj != null
            && Modifier.isProtected(modifiers)
            && ((targetClass = obj.getClass()) != clazz)) {
            // Must match a 2-list of { caller, targetClass }.
            if (cache instanceof Class[]) {
                Class<?>[] cache2 = (Class<?>[]) cache;
                if (cache2[1] == targetClass &&
                    cache2[0] == caller) {
                    return;     // ACCESS IS OK
                }
                // (Test cache[1] first since range check for [1]
                // subsumes range check for [0].)
            }
        } else if (cache == caller) {
            // Non-protected case (or obj.class == this.clazz).
            return;             // ACCESS IS OK
        }

        // If no return, fall through to the slow path.
        slowCheckMemberAccess(caller, clazz, obj, modifiers, targetClass);
    }

    // Keep all this slow stuff out of line:
    void slowCheckMemberAccess(Class<?> caller, Class<?> clazz, Object obj, int modifiers,
                               Class<?> targetClass)
        throws IllegalAccessException
    {
        Reflection.ensureMemberAccess(caller, clazz, obj, modifiers);

        // Success: Update the cache.
        Object cache = ((targetClass == clazz)
                        ? caller
                        : new Class<?>[] { caller, targetClass });

        // Note:  The two cache elements are not volatile,
        // but they are effectively final.  The Java memory model
        // guarantees that the initializing stores for the cache
        // elements will occur before the volatile write.
        securityCheckCache = cache;         // write volatile
    }
}