【Java知识点】Refection获取调用栈信息

    在研究Unsafe类的时候发现了一段有趣的代码，如下：

public static Unsafe getUnsafe() {
    	Class cc = sun.reflect.Reflection.getCallerClass(2); // 获取调用栈信息
    	if (cc.getClassLoader() != null)
    		throw new SecurityException("Unsafe");
    	return theUnsafe;
    }

深入看了下，原来这段代码的目的是为了对Unsafe的安全使用做控制，不允许其他类直接调用Unsafe的静态方法getUnsafe()：(直接调用会抛异常)

Class cc = sun.reflect.Reflection.getCallerClass(2);

这段代码是获取类的调用栈信息，参数i(int 类型)表示栈帧的层次，比如i=0返回Reflection类本身，i=1则返回调用Refelection的类Unsafe，i=2则返回调用Unsafe类的类，这样就可以判断Unsafe是否是直接被调用的。不过我们可以绕过这个限制，使用Java的反射调用完成：

Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
theUnsafe.setAccessible(true);
Unsafe unsafe = (Unsafe) theUnsafe.get(null);

这样就可以使用Unsafe了。

下面是Refection类的源码：

/*
 * %W% %E%
 *
 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package sun.reflect;

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

/** Common utility routines used by both java.lang and
    java.lang.reflect */

public class Reflection {

    /** Used to filter out fields from certain classes from public
        view, where those fields may contain VM-internal objects */
    private static Map/*<Class, String[]>*/ fieldFilterMap = Collections.synchronizedMap(new HashMap());

    /** Returns the class of the method <code>realFramesToSkip</code>
        frames up the stack (zero-based), ignoring frames associated
        with java.lang.reflect.Method.invoke() and its implementation.
        The first frame is that associated with this method, so
        <code>getCallerClass(0)</code> returns the Class object for
        sun.reflect.Reflection. Frames associated with
        java.lang.reflect.Method.invoke() and its implementation are
        completely ignored and do not count toward the number of "real"
        frames skipped. */
    public static native Class getCallerClass(int realFramesToSkip);

    /** Retrieves the access flags written to the class file. For
        inner classes these flags may differ from those returned by
        Class.getModifiers(), which searches the InnerClasses
        attribute to find the source-level access flags. This is used
        instead of Class.getModifiers() for run-time access checks due
        to compatibility reasons; see 4471811. Only the values of the
        low 13 bits (i.e., a mask of 0x1FFF) are guaranteed to be
        valid. */
    private static native int getClassAccessFlags(Class c);

    /** A quick "fast-path" check to try to avoid getCallerClass()
        calls. */
    public static boolean quickCheckMemberAccess(Class memberClass,
                                                 int modifiers)
    {
        return Modifier.isPublic(getClassAccessFlags(memberClass) & modifiers);
    }

    public static void ensureMemberAccess(Class currentClass,
                                          Class memberClass,
                                          Object target,
                                          int modifiers)
        throws IllegalAccessException
    {
        if (currentClass == null || memberClass == null) {
            throw new InternalError();
        }

        if (!verifyMemberAccess(currentClass, memberClass, target, modifiers)) {
            throw new IllegalAccessException("Class " + currentClass.getName() +
                                             " can not access a member of class " +
                                             memberClass.getName() +
                                             " with modifiers \"" +
                                             Modifier.toString(modifiers) +
                                             "\"");
        }
    }

    public static boolean verifyMemberAccess(Class currentClass,
                                             // Declaring class of field
                                             // or method
                                             Class  memberClass,
                                             // May be NULL in case of statics
                                             Object target,
                                             int    modifiers)
    {
        // Verify that currentClass can access a field, method, or
        // constructor of memberClass, where that member's access bits are
        // "modifiers".

        boolean gotIsSameClassPackage = false;
        boolean isSameClassPackage = false;

        if (currentClass == memberClass) {
            // Always succeeds
            return true;
        }

        if (!Modifier.isPublic(getClassAccessFlags(memberClass))) {
            isSameClassPackage = isSameClassPackage(currentClass, memberClass);
            gotIsSameClassPackage = true;
            if (!isSameClassPackage) {
                return false;
            }
        }

        // At this point we know that currentClass can access memberClass.

        if (Modifier.isPublic(modifiers)) {
            return true;
        }

        boolean successSoFar = false;

        if (Modifier.isProtected(modifiers)) {
            // See if currentClass is a subclass of memberClass
            if (isSubclassOf(currentClass, memberClass)) {
                successSoFar = true;
            }
        }

        if (!successSoFar && !Modifier.isPrivate(modifiers)) {
            if (!gotIsSameClassPackage) {
                isSameClassPackage = isSameClassPackage(currentClass,
                                                        memberClass);
                gotIsSameClassPackage = true;
            }

            if (isSameClassPackage) {
                successSoFar = true;
            }
        }

        if (!successSoFar) {
            return false;
        }

        if (Modifier.isProtected(modifiers)) {
            // Additional test for protected members: JLS 6.6.2
            Class targetClass = (target == null ? memberClass : target.getClass());
            if (targetClass != currentClass) {
                if (!gotIsSameClassPackage) {
                    isSameClassPackage = isSameClassPackage(currentClass, memberClass);
                    gotIsSameClassPackage = true;
                }
                if (!isSameClassPackage) {
                    if (!isSubclassOf(targetClass, currentClass)) {
                        return false;
                    }
                }
            }
        }

        return true;
    }
                                    
    private static boolean isSameClassPackage(Class c1, Class c2) {
        return isSameClassPackage(c1.getClassLoader(), c1.getName(),
                                  c2.getClassLoader(), c2.getName());
    }

    /** Returns true if two classes are in the same package; classloader
        and classname information is enough to determine a class's package */
    private static boolean isSameClassPackage(ClassLoader loader1, String name1,
                                              ClassLoader loader2, String name2)
    {
        if (loader1 != loader2) {
            return false;
        } else {
            int lastDot1 = name1.lastIndexOf('.');
            int lastDot2 = name2.lastIndexOf('.');
            if ((lastDot1 == -1) || (lastDot2 == -1)) {
                // One of the two doesn't have a package.  Only return true
                // if the other one also doesn't have a package.
                return (lastDot1 == lastDot2);
            } else {
                int idx1 = 0;
                int idx2 = 0;

                // Skip over '['s
                if (name1.charAt(idx1) == '[') {
                    do {
                        idx1++;
                    } while (name1.charAt(idx1) == '[');
                    if (name1.charAt(idx1) != 'L') {
                        // Something is terribly wrong.  Shouldn't be here.
                        throw new InternalError("Illegal class name " + name1);
                    }
                }
                if (name2.charAt(idx2) == '[') {
                    do {
                        idx2++;
                    } while (name2.charAt(idx2) == '[');
                    if (name2.charAt(idx2) != 'L') {
                        // Something is terribly wrong.  Shouldn't be here.
                        throw new InternalError("Illegal class name " + name2);
                    }
                }

                // Check that package part is identical
                int length1 = lastDot1 - idx1;
                int length2 = lastDot2 - idx2;

                if (length1 != length2) {
                    return false;
                }
                return name1.regionMatches(false, idx1, name2, idx2, length1);
            }
        }
    }

    static boolean isSubclassOf(Class queryClass,
                                Class ofClass)
    {
        while (queryClass != null) {
            if (queryClass == ofClass) {
                return true;
            }
            queryClass = queryClass.getSuperclass();
        }
        return false;
    }

    // fieldNames must contain only interned Strings
    public static void registerFieldsToFilter(Class containingClass,
                                              String[] fieldNames) {
        fieldFilterMap.put(containingClass, fieldNames);
    }

    public static Field[] filterFields(Class containingClass,
                                       Field[] fields) {
        if (fieldFilterMap == null) {
            // Bootstrapping
            return fields;
        }
        String[] filteredFieldNames = (String[]) fieldFilterMap.get(containingClass);
        if (filteredFieldNames == null) {
            return fields;
        }
        int numNewFields = 0;
        for (int srcIdx = 0; srcIdx < fields.length; srcIdx++) {
            boolean shouldSkip = false;
            Field field = fields[srcIdx];
            for (int tmp = 0; tmp < filteredFieldNames.length; tmp++) {
                if (field.getName() == filteredFieldNames[tmp]) {
                    shouldSkip = true;
                    break;
                }
            }
            if (!shouldSkip) {
                ++numNewFields;
            }
        }
        Field[] newFields = new Field[numNewFields];
        int destIdx = 0;
        for (int srcIdx = 0; srcIdx < fields.length; srcIdx++) {
            boolean shouldSkip = false;
            Field field = fields[srcIdx];
            for (int tmp = 0; tmp < filteredFieldNames.length; tmp++) {
                if (field.getName() == filteredFieldNames[tmp]) {
                    shouldSkip = true;
                    break;
                }
            }
            if (!shouldSkip) {
                newFields[destIdx++] = field;
            }
        }
        return newFields;
    }
}