在单例模式这块，我们花了几个篇幅来讲了里面的道道，使用了几种方式来构建了看似无懈可击的单例。但是真的无懈可击吗？下面几篇文章，我们来聊聊对单例模式的攻击以及该如何防御这些攻击。
破坏单例的可能性有哪些
我们知道要破坏单例，则必须创建对象，那么我们顺着这个思路走，创建对象的方式无非就是new，clone，反序列化，以及反射。
- new
单例模式的首要条件就是构造方法私有化，所以new这种方式去破坏单例的可能性是不存在的（在保障线程安全的情况下） - clone
要调用clone方法，那么必须实现Cloneable接口，但是单例模式是不能实现这个接口的，因此排除这种可能性 - 反序列化
这个正是本篇文章要讨论的问题，下面详细进行分析。 - 反射
且移步下篇文章单例模式的攻击之反射
序列化攻击
为方便测试，我们写个最简单的饿汉式单例模式代码，便于以后测试
/**
* @Author: ming.wang
* @Date: 2019/2/20 17:04
* @Description: 为了演示序列化实现Serializable 接口
*/
public class HungrySingleton implements Serializable {
private final static HungrySingleton instance;
static {
instance=new HungrySingleton();
}
private HungrySingleton() {}
public static HungrySingleton getInstance(){
return instance;
}
}
建立一个DestroySingletonTest 测试类，代码如下，简单来说就是采用序列化来破坏单例
/**
* @Author: ming.wang
* @Date: 2019/2/21 16:05
* @Description: 使用反射或反序列化来破坏单例
*/
public class DestroySingletonTest {
public static void main(String[] args) throws IOException, ClassNotFoundException {
//序列化方式破坏单例 测试
serializeDestroyMethod();
}
private static void serializeDestroyMethod() throws IOException, ClassNotFoundException {
HungrySingleton hungrySingleton=null;
HungrySingleton hungrySingleton_new=null;
hungrySingleton=HungrySingleton.getInstance();
ByteArrayOutputStream bos=new ByteArrayOutputStream();
ObjectOutputStream oos=new ObjectOutputStream(bos);
oos.writeObject(hungrySingleton);
ByteArrayInputStream bis=new ByteArrayInputStream(bos.toByteArray());
ObjectInputStream ois=new ObjectInputStream(bis);
hungrySingleton_new= (HungrySingleton) ois.readObject();
System.out.println(hungrySingleton==hungrySingleton_new);
}
}
我们运行程序，结果打印false，显然单例被破坏了。
那么，我们有什么解决办法能抵挡这种序列化破坏呢？
下面我们对HungrySingleton进行小小的修改 ，添加一个方法readResolve()
public class HungrySingleton implements Serializable {
private final static HungrySingleton instance;
...
...
private Object readResolve()
{
return instance;
}
}
我们重新运行程序，发现此时打印的结果是true。显然，这个小小的改动帮我们抵御了序列化对单例的破坏。
下面我们就简单分析一下，为什么这个改动能够起到扭转乾坤的作用。
我们进入ObjectInputStream.readObject()这个方法体内
....
public final Object readObject()
throws IOException, ClassNotFoundException
{
if (enableOverride) {
return readObjectOverride();
}
// if nested read, passHandle contains handle of enclosing object
int outerHandle = passHandle;
try {
Object obj = readObject0(false);
handles.markDependency(outerHandle, passHandle);
ClassNotFoundException ex = handles.lookupException(passHandle);
if (ex != null) {
throw ex;
}
if (depth == 0) {
vlist.doCallbacks();
}
return obj;
} finally {
passHandle = outerHandle;
if (closed && depth == 0) {
clear();
}
}
}
.....
继续跟进 Object obj = readObject0(false);这个方法
....
/**
* Underlying readObject implementation.
*/
private Object readObject0(boolean unshared) throws IOException {
boolean oldMode = bin.getBlockDataMode();
if (oldMode) {
int remain = bin.currentBlockRemaining();
if (remain > 0) {
throw new OptionalDataException(remain);
} else if (defaultDataEnd) {
/*
* Fix for 4360508: stream is currently at the end of a field
* value block written via default serialization; since there
* is no terminating TC_ENDBLOCKDATA tag, simulate
* end-of-custom-data behavior explicitly.
*/
throw new OptionalDataException(true);
}
bin.setBlockDataMode(false);
}
byte tc;
while ((tc = bin.peekByte()) == TC_RESET) {
bin.readByte();
handleReset();
}
depth++;
totalObjectRefs++;
try {
switch (tc) {
case TC_NULL:
return readNull();
case TC_REFERENCE:
return readHandle(unshared);
case TC_CLASS:
return readClass(unshared);
case TC_CLASSDESC:
case TC_PROXYCLASSDESC:
return readClassDesc(unshared);
case TC_STRING:
case TC_LONGSTRING:
return checkResolve(readString(unshared));
case TC_ARRAY:
return checkResolve(readArray(unshared));
case TC_ENUM:
return checkResolve(readEnum(unshared));
case TC_OBJECT:
return checkResolve(readOrdinaryObject(unshared));
case TC_EXCEPTION:
IOException ex = readFatalException();
throw new WriteAbortedException("writing aborted", ex);
case TC_BLOCKDATA:
case TC_BLOCKDATALONG:
if (oldMode) {
bin.setBlockDataMode(true);
bin.peek(); // force header read
throw new OptionalDataException(
bin.currentBlockRemaining());
} else {
throw new StreamCorruptedException(
"unexpected block data");
}
case TC_ENDBLOCKDATA:
if (oldMode) {
throw new OptionalDataException(true);
} else {
throw new StreamCorruptedException(
"unexpected end of block data");
}
default:
throw new StreamCorruptedException(
String.format("invalid type code: %02X", tc));
}
} finally {
depth--;
bin.setBlockDataMode(oldMode);
}
}
....
我们看方法体中的switch 分支，我们会走 case TC_OBJECT:这个分支，那么我们继续跟进，在这个分支下调用了checkResolve(readOrdinaryObject(unshared));，我们着重看readOrdinaryObject(unshared)这个方法，跟进去瞅瞅
....
/**
* Reads and returns "ordinary" (i.e., not a String, Class,
* ObjectStreamClass, array, or enum constant) object, or null if object's
* class is unresolvable (in which case a ClassNotFoundException will be
* associated with object's handle). Sets passHandle to object's assigned
* handle.
*/
private Object readOrdinaryObject(boolean unshared)
throws IOException
{
if (bin.readByte() != TC_OBJECT) {
throw new InternalError();
}
ObjectStreamClass desc = readClassDesc(false);
desc.checkDeserialize();
Class<?> cl = desc.forClass();
if (cl == String.class || cl == Class.class
|| cl == ObjectStreamClass.class) {
throw new InvalidClassException("invalid class descriptor");
}
Object obj;
try {
obj = desc.isInstantiable() ? desc.newInstance() : null;
} catch (Exception ex) {
throw (IOException) new InvalidClassException(
desc.forClass().getName(),
"unable to create instance").initCause(ex);
}
passHandle = handles.assign(unshared ? unsharedMarker : obj);
ClassNotFoundException resolveEx = desc.getResolveException();
if (resolveEx != null) {
handles.markException(passHandle, resolveEx);
}
if (desc.isExternalizable()) {
readExternalData((Externalizable) obj, desc);
} else {
readSerialData(obj, desc);
}
handles.finish(passHandle);
if (obj != null &&
handles.lookupException(passHandle) == null &&
desc.hasReadResolveMethod())
{
Object rep = desc.invokeReadResolve(obj);
if (unshared && rep.getClass().isArray()) {
rep = cloneArray(rep);
}
if (rep != obj) {
// Filter the replacement object
if (rep != null) {
if (rep.getClass().isArray()) {
filterCheck(rep.getClass(), Array.getLength(rep));
} else {
filterCheck(rep.getClass(), -1);
}
}
handles.setObject(passHandle, obj = rep);
}
}
return obj;
}
...
我们抓重点，看下这句代码 obj = desc.isInstantiable() ? desc.newInstance() : null;，我们跟进 desc.isInstantiable()这个方法,看方法注释，简单来说一个类是serializable/externalizable的实例则返回true。
/**
* Returns true if represented class is serializable/externalizable and can
* be instantiated by the serialization runtime--i.e., if it is
* externalizable and defines a public no-arg constructor, or if it is
* non-externalizable and its first non-serializable superclass defines an
* accessible no-arg constructor. Otherwise, returns false.
*/
boolean isInstantiable() {
requireInitialized();
return (cons != null);
}
为true的话，那么obj=desc.newInstance()，通过查看我们知道这个最终是调用的反射机制生成的新的实例。截止到此时，我们可以解释在未做改动之前，生成了新的实例，单例被破坏的真正原因了。
我们接着分析，既然改动之后，成功抵御了单例的破坏，那么后面肯定有相应的代码实现。我们继续看readOrdinaryObject这个方法。往后看，我们发现了这样一句代码，
...
if (obj != null &&
handles.lookupException(passHandle) == null &&
desc.hasReadResolveMethod())
{
Object rep = desc.invokeReadResolve(obj);
...
}
....
接着看desc.hasReadResolveMethod()这个方法,简单来说，就是该类是serializable or externalizable的实例，并且定义了符合要求的readResolve 方法，则返回true
...
/**
* Returns true if represented class is serializable or externalizable and
* defines a conformant readResolve method. Otherwise, returns false.
*/
boolean hasReadResolveMethod() {
requireInitialized();
return (readResolveMethod != null);
}
...
什么是符合要求的readResolve 方法呢？我们搜索readResolve 会发现readResolveMethod = getInheritableMethod( cl, "readResolve", null, Object.class);,符合要求的方法是方法名是readResolve，返回值是Object。
显然定义了符合要求的方法之后，再执行Object rep = desc.invokeReadResolve(obj);反射调用该方法。具体的就是
private Object readResolve()
{
return instance;
}
这样我们就得到了原来的实例而不是新的实例！！
结论
对于序列化破坏单例，我们的解决方案就是，在单例代码中，增加以下代码
private Object readResolve()
{
return instance;
}
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