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how can i use java scanner with lazy singeleton?

i want to make 2 input scanner in java with Lazy singeleton algorithm..
and print it..
the question is : write a java program to get username and password then print it(with Lazy singeleton algorithm)
import java.util.Scanner;
public class lazy1 {
String a1;
String a2;
private static lazy1 Instance;
public synchronized static lazy1 getInstance() {
if (Instance == null) {
Instance = new lazy1();
}
return Instance;
}
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("uesrname");
String a1 = sc.nextLine();
System.out.println("password");
String a2 = sc.nextLine();
System.out.println("username : "+ a1);
System.out.println("password : "+ a2);
}
}

lazy singletons are completely useless*. The aim is, presumably, that the singleton is not 'initialized' or 'takes up memory'. But, good news! It won't. Not until some code touches the singleton class, at which point, well, you needed that initialization anyway. Thus:
public class Lazy1 {
private static final Lazy1 INSTANCE = new Lazy1();
private Lazy1() {} // prevent instantiation by anybody else
public static Lazy1 getInstance() { return INSTANCE; }
}
Does what you want: It is simpler, not buggy, and fast. In contrast to your take, which is quite slow (a synchronized lock every time, even on hour 100, after a million calls to getinstance. However, remove the synchronized and you end up in a buggy scenario. You can try to get around that in turn with volatiles and double locking, but now you have quite complicated code that is almost impossible to test, and which still cannot outperform the fast locking that the classloader itself gives you).
You may think this is non-lazy, but that's false. Try it: print something in that constructor and observe when it prints. It'll print only when the first getInstance() is ever invoked.
Given that it is a singleton, if you need some property (such as 'a scanner') to operate, you have only two options:
The singleton is in an invalid state until it is initialized
All methods that need this state check if the state is valid and throw if it is not:
public class Lazy1 {
private static final Lazy1 INSTANCE = new Lazy1();
private Lazy1() {} // prevent instantiation by anybody else
public static Lazy1 getInstance() { return INSTANCE; }
private Scanner scanner;
public void menu() {
if (scanner == null) throw new IllegalStateException("Uninitialized");
// ....
}
public void init(Scanner s) {
this.scanner = s;
}
}
Every method that needs the 'scanner state' should first check, then throw.
The singleton doesn't have state. Instead, its methods take the required state as parameter
Your Lazy1 doesn't have, say, public void menu(). It has public void menu(Scanner s) {} - every time any code calls it, it passes scanner along.
*) There is actually a point, but only if some code refers to the Lazy1 class without getting the singleton. If you're doing that, you probably need to fix your code; that'd be rather weird.

Singleton & Multithreading in Java

What is the preferred way to work with Singleton class in multithreaded environment?
Suppose if I have 3 threads, and all of them try to access getInstance() method of singleton class at the same time -
What would happen if no synchronization is maintained?
Is it good practice to use synchronized getInstance() method or use synchronized block inside getInstance().
Please advise if there is any other way out.

If you're talking about threadsafe, lazy initialization of the singleton, here is a cool code pattern to use that accomplishes 100% threadsafe lazy initialization without any synchronization code:
public class MySingleton {
private static class MyWrapper {
static MySingleton INSTANCE = new MySingleton();
}
private MySingleton () {}
public static MySingleton getInstance() {
return MyWrapper.INSTANCE;
}
}
This will instantiate the singleton only when getInstance() is called, and it's 100% threadsafe! It's a classic.
It works because the class loader has its own synchronization for handling static initialization of classes: You are guaranteed that all static initialization has completed before the class is used, and in this code the class is only used within the getInstance() method, so that's when the class loaded loads the inner class.
As an aside, I look forward to the day when a #Singleton annotation exists that handles such issues.
Edited:
A particular disbeliever has claimed that the wrapper class "does nothing". Here is proof that it does matter, albeit under special circumstances.
The basic difference is that with the wrapper class version, the singleton instance is created when the wrapper class is loaded, which when the first call the getInstance() is made, but with the non-wrapped version - ie a simple static initialization - the instance is created when the main class is loaded.
If you have only simple invocation of the getInstance() method, then there is almost no difference - the difference would be that all other sttic initialization would have completed before the instance is created when using the wrapped version, but this is easily dealt with by simply having the static instance variable listed last in the source.
However, if you are loading the class by name, the story is quite different. Invoking Class.forName(className) on a class cuasing static initialization to occur, so if the singleton class to be used is a property of your server, with the simple version the static instance will be created when Class.forName() is called, not when getInstance() is called. I admit this is a little contrived, as you need to use reflection to get the instance, but nevertheless here's some complete working code that demonstrates my contention (each of the following classes is a top-level class):
public abstract class BaseSingleton {
private long createdAt = System.currentTimeMillis();
public String toString() {
return getClass().getSimpleName() + " was created " + (System.currentTimeMillis() - createdAt) + " ms ago";
}
}
public class EagerSingleton extends BaseSingleton {
private static final EagerSingleton INSTANCE = new EagerSingleton();
public static EagerSingleton getInstance() {
return INSTANCE;
}
}
public class LazySingleton extends BaseSingleton {
private static class Loader {
static final LazySingleton INSTANCE = new LazySingleton();
}
public static LazySingleton getInstance() {
return Loader.INSTANCE;
}
}
And the main:
public static void main(String[] args) throws Exception {
// Load the class - assume the name comes from a system property etc
Class<? extends BaseSingleton> lazyClazz = (Class<? extends BaseSingleton>) Class.forName("com.mypackage.LazySingleton");
Class<? extends BaseSingleton> eagerClazz = (Class<? extends BaseSingleton>) Class.forName("com.mypackage.EagerSingleton");
Thread.sleep(1000); // Introduce some delay between loading class and calling getInstance()
// Invoke the getInstace method on the class
BaseSingleton lazySingleton = (BaseSingleton) lazyClazz.getMethod("getInstance").invoke(lazyClazz);
BaseSingleton eagerSingleton = (BaseSingleton) eagerClazz.getMethod("getInstance").invoke(eagerClazz);
System.out.println(lazySingleton);
System.out.println(eagerSingleton);
}
Output:
LazySingleton was created 0 ms ago
EagerSingleton was created 1001 ms ago
As you can see, the non-wrapped, simple implementation is created when Class.forName() is called, which may be before the static initialization is ready to be executed.

The task is non-trivial in theory, given that you want to make it truly thread safe.
A very nice paper on the matter is found # IBM
Just getting the singleton does not need any sync, since it's just a read. So, just synchronize the setting of the Sync would do. Unless two treads try to create the singleton at start up at the same time, then you need to make sure check if the instance is set twice (one outside and one inside the sync) to avoid resetting the instance in a worst case scenario.
Then you might need to take into account how JIT (Just-in-time) compilers handle out-of-order writes. This code will be somewhat near the solution, although won't be 100% thread safe anyway:
public static Singleton getInstance() {
if (instance == null) {
synchronized(Singleton.class) {
Singleton inst = instance;
if (inst == null) {
synchronized(Singleton.class) {
instance = new Singleton();
}
}
}
}
return instance;
}
So, you should perhaps resort to something less lazy:
class Singleton {
private static Singleton instance = new Singleton();
private Singleton() { }
public static Singleton getInstance() {
return instance;
}
}
Or, a bit more bloated, but a more flexible way is to avoid using static singletons and use an injection framework such as Spring to manage instantiation of "singleton-ish" objects (and you can configure lazy initialization).

You need synchronization inside getInstance only if you initialize your singleton lazily. If you could create an instance before the threads are started, you can drop synchronization in the getter, because the reference becomes immutable. Of course if the singleton object itself is mutable, you would need to synchronize its methods which access information that can be changed concurrently.

This question really depends on how and when your instance is created. If your getInstance method lazily initializes:
if(instance == null){
instance = new Instance();
}
return instance
Then you must synchronize or you could end up with multiple instances. This problem is usually treated in talks on Double Checked Locking.
Otherwise if you create a static instance up front
private static Instance INSTANCE = new Instance();
then no synchronization of the getInstance() method is necessary.

The best way as described in effective java is:
public class Singelton {
private static final Singelton singleObject = new Singelton();
public Singelton getInstance(){
return singleObject;
}
}
No need of synchronization.

Nobody uses Enums as suggested in Effective Java?

If you are sure that your java runtime is using the new JMM (Java memory model, probably newer than 5.0), double check lock is just fine, but add a volatile in front of instance. Otherwise, you'd better use static internal class as Bohemian said, or Enum in 'Effective Java' as Florian Salihovic said.

For simplicity, I think using enum class is a better way. We don't need to do any synchronization. Java by construct, always ensure that there is only one constant created, no matter how many threads are trying to access it.
FYI, In some case you need to swap out singleton with other implementation. Then we need to modify class, which is violation of Open Close principal.Problem with singleton is, you can't extend the class because of having private constructor. So, it's a better practice that client is talking via interface.
Implementation of Singleton with enum class and Interface:
Client.java
public class Client{
public static void main(String args[]){
SingletonIface instance = EnumSingleton.INSTANCE;
instance.operationOnInstance("1");
}
}
SingletonIface.java
public interface SingletonIface {
public void operationOnInstance(String newState);
}
EnumSingleton.java
public enum EnumSingleton implements SingletonIface{
INSTANCE;
#Override
public void operationOnInstance(String newState) {
System.out.println("I am Enum based Singleton");
}
}

The Answer is already accepted here, But i would like to share the test to answer your 1st question.
What would happen if no synchronization is maintained?
Here is the SingletonTest class which will be completely disaster when you run in multi Threaded Environment.
/**
* #author MILAN
*/
public class SingletonTest
{
private static final int PROCESSOR_COUNT = Runtime.getRuntime().availableProcessors();
private static final Thread[] THREADS = new Thread[PROCESSOR_COUNT];
private static int instancesCount = 0;
private static SingletonTest instance = null;
/**
* private constructor to prevent Creation of Object from Outside of the
* This class.
*/
private SingletonTest()
{
}
/**
* return the instance only if it does not exist
*/
public static SingletonTest getInstance()
{
if (instance == null)
{
instancesCount++;
instance = new SingletonTest();
}
return instance;
}
/**
* reset instancesCount and instance.
*/
private static void reset()
{
instancesCount = 0;
instance = null;
}
/**
* validate system to run the test
*/
private static void validate()
{
if (SingletonTest.PROCESSOR_COUNT < 2)
{
System.out.print("PROCESSOR_COUNT Must be >= 2 to Run the test.");
System.exit(0);
}
}
public static void main(String... args)
{
validate();
System.out.printf("Summary :: PROCESSOR_COUNT %s, Running Test with %s of Threads. %n", PROCESSOR_COUNT, PROCESSOR_COUNT);
long currentMili = System.currentTimeMillis();
int testCount = 0;
do
{
reset();
for (int i = 0; i < PROCESSOR_COUNT; i++)
THREADS[i] = new Thread(SingletonTest::getInstance);
for (int i = 0; i < PROCESSOR_COUNT; i++)
THREADS[i].start();
for (int i = 0; i < PROCESSOR_COUNT; i++)
try
{
THREADS[i].join();
}
catch (InterruptedException e)
{
e.printStackTrace();
Thread.currentThread().interrupt();
}
testCount++;
}
while (instancesCount <= 1 && testCount < Integer.MAX_VALUE);
System.out.printf("Singleton Pattern is broken after %d try. %nNumber of instances count is %d. %nTest duration %dms", testCount, instancesCount, System.currentTimeMillis() - currentMili);
}
}
Output of the program is clearly shows that you need handle this using getInstance as synchronized or add synchronized lock enclosing new SingletonTest.
Summary :: PROCESSOR_COUNT 32, Running Test with 32 of Threads.
Singleton Pattern is broken after 133 try.
Number of instance count is 30.
Test duration 500ms

How to implement thread-safe lazy initialization?

What are some recommended approaches to achieving thread-safe lazy initialization? For instance,
// Not thread-safe
public Foo getInstance(){
if(INSTANCE == null){
INSTANCE = new Foo();
}
return INSTANCE;
}

For singletons there is an elegant solution by delegating the task to the JVM code for static initialization.
public class Something {
private Something() {
}
private static class LazyHolder {
public static final Something INSTANCE = new Something();
}
public static Something getInstance() {
return LazyHolder.INSTANCE;
}
}
see
http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom
and this blog post of Crazy Bob Lee
http://blog.crazybob.org/2007/01/lazy-loading-singletons.html

If you're using Apache Commons Lang, then you can use one of the variations of ConcurrentInitializer like LazyInitializer.
Example:
ConcurrentInitializer<Foo> lazyInitializer = new LazyInitializer<Foo>() {
#Override
protected Foo initialize() throws ConcurrentException {
return new Foo();
}
};
You can now safely get Foo (gets initialized only once):
Foo instance = lazyInitializer.get();
If you're using Google's Guava:
Supplier<Foo> fooSupplier = Suppliers.memoize(new Supplier<Foo>() {
public Foo get() {
return new Foo();
}
});
Then call it by Foo f = fooSupplier.get();
From Suppliers.memoize javadoc:
Returns a supplier which caches the instance retrieved during the first call to get() and returns that value on subsequent calls to get(). The returned supplier is thread-safe. The delegate's get() method will be invoked at most once. If delegate is an instance created by an earlier call to memoize, it is returned directly.

This can be done in lock-free manner by using AtomicReference as instance holder:
// in class declaration
private AtomicReference<Foo> instance = new AtomicReference<>(null);
public Foo getInstance() {
Foo foo = instance.get();
if (foo == null) {
foo = new Foo(); // create and initialize actual instance
if (instance.compareAndSet(null, foo)) // CAS succeeded
return foo;
else // CAS failed: other thread set an object
return instance.get();
} else {
return foo;
}
}
Main disadvantage here is that multiple threads can concurrently instantiate two or more Foo objects, and only one will be lucky to be set up, so if instantiation requires I/O or another shared resource, this method may not be suitable.
At the other side, this approach is lock-free and wait-free: if one thread which first entered this method is stuck, it won't affect execution of others.

The easiest way is to use a static inner holder class :
public class Singleton {
private Singleton() {
}
public static Singleton getInstance() {
return Holder.INSTANCE;
}
private static class Holder {
private static final Singleton INSTANCE = new Singleton();
}
}

class Foo {
private volatile Helper helper = null;
public Helper getHelper() {
if (helper == null) {
synchronized(this) {
if (helper == null) {
helper = new Helper();
}
}
}
return helper;
}
This is called double checking!
Check this http://jeremymanson.blogspot.com/2008/05/double-checked-locking.html

If you use lombok in your project, you can use a feature described here.
You just create a field, annotate it with #Getter(lazy=true) and add initialization, like this:
#Getter(lazy=true)
private final Foo instance = new Foo();
You'll have to reference field only with getter (see notes in lombok docs), but in most cases that's what we need.

Here is one more approach which is based on one-time-executor semantic.
The full solution with bunch of usage examples can be found on github (https://github.com/ManasjyotiSharma/java_lazy_init). Here is the crux of it:
“One Time Executor” semantic as the name suggests has below properties:
A wrapper object which wraps a function F. In current context F is a function/lambda expression which holds the initialization/de-initialization code.
The wrapper provides an execute method which behaves as:
Calls the function F the first time execute is called and caches the output of F.
If 2 or more threads call execute concurrently, only one “gets in” and the others block till the one which “got in” is done.
For all other/future invocations of execute, it does not call F rather simply returns the previously cached output.
The cached output can be safely accessed from outside of the initialization context.
This can be used for initialization as well as non-idempotent de-initialization too.
import java.util.Objects;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Function;
/**
* When execute is called, it is guaranteed that the input function will be applied exactly once.
* Further it's also guaranteed that execute will return only when the input function was applied
* by the calling thread or some other thread OR if the calling thread is interrupted.
*/
public class OneTimeExecutor<T, R> {
private final Function<T, R> function;
private final AtomicBoolean preGuard;
private final CountDownLatch postGuard;
private final AtomicReference<R> value;
public OneTimeExecutor(Function<T, R> function) {
Objects.requireNonNull(function, "function cannot be null");
this.function = function;
this.preGuard = new AtomicBoolean(false);
this.postGuard = new CountDownLatch(1);
this.value = new AtomicReference<R>();
}
public R execute(T input) throws InterruptedException {
if (preGuard.compareAndSet(false, true)) {
try {
value.set(function.apply(input));
} finally {
postGuard.countDown();
}
} else if (postGuard.getCount() != 0) {
postGuard.await();
}
return value();
}
public boolean executed() {
return (preGuard.get() && postGuard.getCount() == 0);
}
public R value() {
return value.get();
}
}
Here is a sample usage:
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.nio.charset.StandardCharsets;
/*
* For the sake of this example, assume that creating a PrintWriter is a costly operation and we'd want to lazily initialize it.
* Further assume that the cleanup/close implementation is non-idempotent. In other words, just like initialization, the
* de-initialization should also happen once and only once.
*/
public class NonSingletonSampleB {
private final OneTimeExecutor<File, PrintWriter> initializer = new OneTimeExecutor<>(
(File configFile) -> {
try {
FileOutputStream fos = new FileOutputStream(configFile);
OutputStreamWriter osw = new OutputStreamWriter(fos, StandardCharsets.UTF_8);
BufferedWriter bw = new BufferedWriter(osw);
PrintWriter pw = new PrintWriter(bw);
return pw;
} catch (IOException e) {
e.printStackTrace();
throw new RuntimeException(e);
}
}
);
private final OneTimeExecutor<Void, Void> deinitializer = new OneTimeExecutor<>(
(Void v) -> {
if (initializer.executed() && null != initializer.value()) {
initializer.value().close();
}
return null;
}
);
private final File file;
public NonSingletonSampleB(File file) {
this.file = file;
}
public void doSomething() throws Exception {
// Create one-and-only-one instance of PrintWriter only when someone calls doSomething().
PrintWriter pw = initializer.execute(file);
// Application logic goes here, say write something to the file using the PrintWriter.
}
public void close() throws Exception {
// non-idempotent close, the de-initialization lambda is invoked only once.
deinitializer.execute(null);
}
}
For few more examples (e.g. singleton initialization which requires some data available only at run-time thus unable to instantiate it in a static block) please refer to the github link mentioned above.

Thinking about lazy initialization, I would expect getting a "almost real" object that just decorates the still not initialized object.
When the first method is being invoked, the instance within the decorated interface will be initialized.
* Because of the Proxy usage, the initiated object must implement the passed interface.
* The difference from other solutions is the encapsulation of the initiation from the usage. You start working directly with DataSource as if it was initialized. It will be initialized on the first method's invocation.
Usage:
DataSource ds = LazyLoadDecorator.create(dsSupplier, DataSource.class)
Behind the scenes:
public class LazyLoadDecorator<T> implements InvocationHandler {
private final Object syncLock = new Object();
protected volatile T inner;
private Supplier<T> supplier;
private LazyLoadDecorator(Supplier<T> supplier) {
this.supplier = supplier;
}
#Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (inner == null) {
synchronized (syncLock) {
if (inner == null) {
inner = load();
}
}
}
return method.invoke(inner, args);
}
protected T load() {
return supplier.get();
}
#SuppressWarnings("unchecked")
public static <T> T create(Supplier<T> factory, Class<T> clazz) {
return (T) Proxy.newProxyInstance(LazyLoadDecorator.class.getClassLoader(),
new Class[] {clazz},
new LazyLoadDecorator<>(factory));
}
}

Put the code in a synchronized block with some suitable lock. There are some other highly specialist techniques, but I'd suggest avoiding those unless absolutely necessary.
Also you've used SHOUTY case, which tends to indicate a static but an instance method. If it is really static, I suggest you make sure it isn't in any way mutable. If it's just an expensive to create static immutable, then class loading is lazy anyway. You may want to move it to a different (possibly nested) class to delay creation to the absolute last possible moment.

Depending on what you try to achieve:
If you want all Threads to share the same instance, you can make the method synchronized. This will be sufficient
If you want to make a separate INSTANCE for each Thread, you should use java.lang.ThreadLocal

With Java 8 we can achieve lazy initialization with thread safety. If we have Holder class and it needs some heavy resources then we can lazy load the heavy resource like this.
public class Holder {
private Supplier<Heavy> heavy = () -> createAndCacheHeavy();
private synchronized Heavy createAndCacheHeavy() {
class HeavyFactory implements Supplier<Heavy> {
private final Heavy heavyInstance = new Heavy();
#Override
public Heavy get() {
return heavyInstance;
}
}
if (!HeavyFactory.class.isInstance(heavy)) {
heavy = new HeavyFactory();
}
return heavy.get();
}
public Heavy getHeavy() {
return heavy.get();
}
}
public class Heavy {
public Heavy() {
System.out.println("creating heavy");
}
}

Basing this answer on #Alexsalauyou's one I thought if it could be possible to implement a solution that does not call multiple instances.
In principle my solution may be a little bit slower (very very little), but it is definitely friendlier to the processor, and the garbage collector.
The idea is that you must first use a container which could hold an "int" value PLUS the generic you want to instance.
static class Container<T> {
final int i;
final T val;
//constructor here
}
Let this container's fields be final for concurrency purposes.
The LazyInit<T> class, must have an AtomicReference of this container.
AtomicReference<Container<T>> ref;
LazyInit must define phase processes as private static int constants:
private static final int NULL_PHASE = -1, CREATING_PHASE = 0, CREATED = 1;
private final Container<T> NULL = new Container<>(NULL_PHASE, null),
CREATING = new Container<>(CREATING_PHASE, null);
NULL and CREATING containers can be made static and upgraded to <?> to make things lighter, then one could grab them with a casting private static method.
The AtomicReference must be initialized as NULL:
private final AtomicReference<Container<T>> ref = new AtomicReference<>(getNull());
Finally the get() method would look like this:
#Override
public T get() {
Container<T> prev;
while ((prev = ref.get()).i < CREATED) {
if (ref.compareAndSet(getNull(), getCreating())) {
T res = builder.get();
ref.set(new Container<>(CREATED, res));
return res;
}
}
return prev.value;
}

Try to defined the method which gets an instance as synchronized:
public synchronized Foo getInstance(){
if(INSTANCE == null){
INSTANCE = new Foo();
}
return INSTANCE;
}
Or use a variable:
private static final String LOCK = "LOCK";
public synchronized Foo getInstance(){
synchronized(LOCK){
if(INSTANCE == null){
INSTANCE = new Foo();
}
}
return INSTANCE;
}

Per thread singleton pattern

In my work I stumbled upon such a design issue:
I need one instance of a Manager class per thread
These instances should be globally accessible, like in the singleton pattern via a static function
Each thread might need to initialize its instance with different arguments
The lifetime of these instances should be controllable, sometimes it would be beneficiary to remove an instance and allow GC to collect it
The first two points would make it a 'per thread singleton' if such a thing exists.
This is what I came up with (the code is simplified, I've omitted safety checks and so on):
public class Manager {
private final static ThreadLocal<Manager> local = new ThreadLocal<Manager>();
private int x;
Manager(int argument) { x = argument; }
public static void start(int argument) { local.set(new Manager(argument); }
public static void clean() { local.remove(); }
private void doSomething1() { x++; .... }
private int doSomething2() { if (--x == 0) clean(); ... }
public static void function1() { local.get().doSomething1(); }
public static int function2() { return local.get().doSomething2(); }
}
As you can see the clean function can be also called from within the private methods.
Also notice that through the use of static functions the reference to the instance is never leaked, so instances assigned to different threads won't get mixed.
This works quite ok, but then I got another requirement:
Different threads may need to utilize different implementations of Manager class
So I defined an interface:
public interface ManagerHandler {
void method1();
int method2();
}
And modified the Manager class:
public class Manager {
private final static ThreadLocal<ManagerHandler> local = new ThreadLocal<ManagerHandler>();
public static void start(int argument) {
ManagerHandler handler;
// depending on the context initialize handler to whatever class it is necessary
local.set(handler);
}
public static void clean() { local.remove(); }
public static void function1() { local.get().method1(); }
public static int function2() { return local.get().method2(); }
}
An example implementation would look like this:
public class ExampleManagerImplementation implements ManagerHandler {
private int x;
public ExampleManagerImplementation(int argument) { x = argument; }
public void method1() { x++; .... }
public int method2() { if (--x == 0) Manager.clean(); ... }
}
Manager class works here as a facade, forwarding all the calls to the appropriate handler. There is one big issue with this approach: I need to define all the functions both in the Manager class and in the ManagerHandler interface. Unfurtunately Manager class can't implement ManagerHandler interface, because it has static functions rather than methods.
The question is: can you think of a better/easier way to accomplish all the goals I've listed above that would be free of this issue?

There is not much you can do, as you basically need to proxy interface methods through static methods. I could only think of two ways to achieve the same functionality differently:
If you're using a DI framework, you can get rid of the static Manager and use an injected implementation of ManagerHandler which will contain the ThreadLocal.
Generate (as in 'bytecode generation') the static ManagerAccess class using the methods found in the ManagerHandler interface.
Personally, I wouldn't think of having the static ManagerAccess class (which contains the ThreadLocal) around as a serious design issue. At least as long as it keeps to its own set of responsibilities (accessing thread-scoped instances and proxying calls) and doesn't venture anywhere else.

If you're going with this design, is it necessary for Manager to totally hide ManagerHandler interface, or could you expose it so you don't have to delegate every method?
class Manager {
public static ManagerHandler getHandler() { return local.get(); }
}

The trick for creating a singleton per thread class is to use ThreadStatic attribute on your private static _current field which makes it scoped by thread. In this way, the _current field will be stored inside thread memory which is not accessible for the other threads and not shared memory of AppDomain. So, it will be available only in the scope of the thread. On the other hand, the Current property is accessible across all threads in that AppDomain but when it is called it will return the correct instance for that thread. Here is the code that you need:
public sealed class Manager
{
// As you are using the ThreadStatic here you cannot
// call the static constructor or use the Lazy implimentation for
// thread-safty and you have to use the old fashin Lock and anti-pattern.
private static readonly object _criticalArea = new object();
[ThreadStatic]
private static Manager _current;
public static Manager Current
{
get
{
if (_current == null)
{
lock (_criticalArea)
{
if (_current == null)
{
_current = new Manager();
}
}
}
return _current;
}
}
private Manager()
{
}
public string WhatThreadIsThis { get; set; }
}
[TestClass]
public class SingeltonPerThreadTest
{
private readonly EventWaitHandle _threadHandler = new EventWaitHandle(false, EventResetMode.AutoReset);
private string _sharedMemory = "I am the shared memory and yet in main thread :(";
[TestMethod]
public void TestSingeltonPerThread()
{
// Creates a _current for main thread.
Manager.Current.WhatThreadIsThis = "I am the main thread :)";
// Start another thread.
(new Thread(CallTheThreadBaseSingelton)).Start();
// Wait for it to be finished.
_threadHandler.WaitOne();
Assert.AreEqual("I am the main thread :)", Manager.Current.WhatThreadIsThis, "I am not the main thread :( ");
Assert.AreEqual("I am the other thread ;)", _sharedMemory, _sharedMemory);
}
private void CallTheThreadBaseSingelton()
{
// Creates a _current for this thread (this thread is the other one :)) ).
Manager.Current.WhatThreadIsThis = "I am the other thread ;)";
_sharedMemory = Manager.Current.WhatThreadIsThis;
_threadHandler.Set();
}
}
Cheers.

Is the following utility class thread-safe?

First let's look at the utility class (most javadoc has been removed to simply the example):
public class ApplicationContextUtils {
/**
* The application context; care should be taken to ensure that 1) this
* variable is assigned exactly once (in the
* {#link #setContext(ApplicationContext)} method, 2) the context is never
* reassigned to {#code null}, 3) access to the field is thread-safe (no race
* conditions can occur)
*/
private static ApplicationContext context = null;
public static ApplicationContext getContext() {
if (!isInitialized()) {
throw new IllegalStateException(
"Context not initialized yet! (Has the "
+ "ApplicationContextProviderBean definition been configured "
+ "properly and has the web application finished "
+ "loading before you invoked this method?)");
}
return context;
}
public static boolean isInitialized() {
return context == null;
}
#SuppressWarnings("unchecked")
public static <T> T getBean(final String name, final Class<T> requiredType) {
if (requiredType == null) {
throw new IllegalArgumentException("requiredType is null");
}
return (T) getContext().getBean(name, requiredType);
}
static synchronized void setContext(final ApplicationContext theContext) {
if (theContext == null) {
throw new IllegalArgumentException("theContext is null");
}
if (context != null) {
throw new IllegalStateException(
"ApplicationContext already initialized: it cannot be done twice!");
}
context = theContext;
}
private ApplicationContextUtils() {
throw new AssertionError(); // NON-INSTANTIABLE UTILITY CLASS
}
}
Finally, there is the following helper Spring managed bean that actually calls the 'setContext' method:
public final class ApplicationContextProviderBean implements
ApplicationContextAware {
public void setApplicationContext(
final ApplicationContext applicationContext) throws BeansException {
ApplicationContextUtils.setContext(applicationContext);
}
}
Spring will call the setApplicationContext method once after the app is started. Assuming a nincompoop has not previously called ApplicationContextUtils.setContext(), that should lock in the reference to the context in the utility class, allowing calls to getContext() to success (meaning that isInitialized() returns true).
I just want to know if this class violates any principles of good coding practices, with respect to thread safety in particular (but other stupidities found are welcome).
Thanks for helping me to become a better programmer, StackOverflow!
Regards,
LES
P.S. I didn't go into why I need this utility class - let it suffice that I indeed do have a legitimate need to access it from a static context anywhere in the application (after the spring context has loaded, of course).

No. It's not thread safe.
Writes to the context class variable are not guaranteed to be visible to threads that read that variable through getContext().
At the very least, declare context to be volatile. Ideally, redefine context as an AtomicReference, set through a call like this:
if(!context.compareAndSet(null, theContext))
throw new IllegalStateException("The context is already set.");
Here's a more complete example:
public class ApplicationContextUtils {
private static final AtomicReference<ApplicationContext> context =
new AtomicReference<ApplicationContext>();
public static ApplicationContext getContext() {
ApplicationContext ctx = context.get();
if (ctx == null)
throw new IllegalStateException();
return ctx;
}
public static boolean isInitialized() {
return context.get() == null;
}
static void setContext(final ApplicationContext ctx) {
if (ctx == null)
throw new IllegalArgumentException();
if (!context.compareAndSet(null, ctx))
throw new IllegalStateException();
}
public static <T> T getBean(final String name, final Class<T> type) {
if (type == null)
throw new IllegalArgumentException();
return type.cast(getContext().getBean(name, type));
}
private ApplicationContextUtils() {
throw new AssertionError();
}
}
Note that in addition to thread safety, this also provides type safety, taking advantage of the Class instance passed into the getBean() method.
I'm not sure how you plan to use the isInitialized() method; it doesn't seem very useful to me, since as soon as you call it, the condition could change, and you don't have a good way to be notified.

Spring already has a class called ContextSingletonBeanFactoryLocator that wires up static access to the ApplicationContext for you. At the least, using this class might save you the trouble of having to worry about if your custom approach is thread safe.
However it's a little confusing to use this class at first, since there's a little bit of indirection going on. You can take a look at this blog post for more information on how this call works.