单例设计模式
单例设计模式的核心思想是，程序运行中只允许某个类只有一个实例
序列化破坏单例模式
首先写一个单例的类
public class Student implements Serializable {
private static Student student;
private Student() {
}
public static Student get() {
if (student == null) {
synchronized (Student.class) {
if (student == null) {
student = new Student();
}
}
}
return student;
}
}
测试Student是否为单例
public static void main(String[] args) throws Exception {
Student oldStudent = Student.get();
System.out.println("序列化之前：" + (oldStudent == Student.get()));
}输出结果为true，说明通过get()方法获取到的两个Student对象是同一对象，该类确实是单例。
序列化进行测试
public static void main(String[] args) throws Exception {
ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream("tempFile"));
objectOutputStream.writeObject(Student.get());
ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream(new File("tempFile")));
Student student = (Student) objectInputStream.readObject();
System.out.println("序列化之后：" + (student == Student.get()));
}输出结果为false。
既然单例模式只允许该类只能有一个实例，那么为什么两个Student对象不是同一个对象？让我们再深入分析一下
对象的序列化过程通过ObjectOutputStream和ObjectInputputStream来实现的，那么带着刚刚的问题，分析一下ObjectInputputStream 的readObject方法执行情况到底是怎样的
readObject源码
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();
}
}
}我们再进入到readObject0方法中
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);
}
}最下面，读取并返回对象的逻辑是在 TC_OBJECT的case中，也就是调用readOrdinaryObject的方法
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;
}直接定位到核心代码
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);
}isInstantiable：如果一个类可以在运行时实例化，就返回true
newInstance：使用反射调用无参构造创建一个对象。
现在已经很明确了，readObject方法是在运行时调用无参构造方法重新创建了对象。即使是私有的方法也可以通过反射去调用，因此私有构造并不能防止用户去创建第二个该类的实例
那么应该如何防止单例模式被破坏呢，只需要在Student类里编写一个readResolve方法
private Object readResolve() {
return student;
}我们回到此前的代码，继续阅读readOrdinaryObject
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);
}
}hasReadResolveMethod：如果实现了序列化的类中包含readResolve方法就返回true
invokeReadResolve：通过反射调用readResolve方法
至此，原理很清楚了，只需要在类中编写一个readResolve方法，就可以防止在序列化过程中破坏单例模式
