I see the following contruct for a mutable class:
public class Doubtful
{
public static Doubtful getInstance()
{
DoubtfulContext doubtfulcontext;//LOCAL HEAP VARIABLE
//...
doubtfulcontext = new DoubtfulContext(s1, new PrincipalName(s),
DefaultConfig.getInstance());
//...
doubtfulcontext.setDoubtfulStore(new DoubtfulStore(new File(s2)));
doubtfulcontext.setKeyTab(...);
doubtfulcontext.setSupportedEncryptionTypes(ai);
//...
return new Doubtful(doubtfulcontext);
}
// ...
}
While Doubtful may be non-mutable,but DoubtContext is definitely mutable.
Is this thread-safe?
What is the relevance of a local heap variable here?
Local variables are confined to the executing thread.They exist on the executing thread's stack and are not accessible to other threads. And this makes the execution of getInstance method thread safe.
As you have said Doubtful is immutable, and that makes it thread safe: multiple threads can work with the same Doubtful instance without effecting others working with the same Doubtful instance. Because the threads cannot change the instance variables (Doubtful is immutable) and method local variables are confined to the executing thread.
Now DoubtfulContext is mutable and you are publishing a reference to the DoubtfulContext instance which is created locally in the method getInstance:
doubtfulcontext = new DoubtfulContext(s1, new PrincipalName(s),
DefaultConfig.getInstance());
...
return new Doubtful(doubtfulcontext);//Publishes the reference to DoubtfulContext
which would violate the stack confinement. And there is a possibility that multiple threads can get access to the shared, mutable data of the same DoubtfulContext instance. If DoubtfulContext is a non-thread-safe object, then this would break your program.
Consider a thread T1 that invokes getInstance to get an instance of Doubtful and after that it might share the DoubtfulContext reference (that came along with Doubtful) with other threads:
1. Doubtful doubtful = Doubtful.getInstance();
2. DoubtfulContext doubtfulContext = doubtful.getDoubtfulContext();
3. new Thread(new SomeRunnable(doubtfulContext)).start();
4. doubtfulContext.chnageSomeState();
At line no 3, it creates a new thread of execution with the DoubtfulContext. Now two threads have the same DoubtfulContext. If DoubtfulContext is non-thread-safe (having non-synchronized access to instance variables), then this would break the thread safety of the program.
This construction looks threadsafe, if there is no method or function to access the doubtfulcontext elsewhere in the class (and if the doubtfulcontext is not modified either), and if... Basically if you use it right, it is threadsafe.
There are a lot of ifs in that sentence. It would be preferable to make the DoubtfulContext non-mutable also.
It's not clear what this code does to say for certain if it is the right choice. The question of thread safety is a question of what is mutable and if that object will ever be seen by more than one thread. Creating a new object that is stateful, but will only ever be seen by one thread is thread-safe. Creating an immutable object with all immutable members is thread-safe whether you return a new one or the same one repeatedly.
If you have mutable state, you have to know if the object will be seen by more than one thread. If yes, then you need to take measures to ensure that the mutable things are thread-safe.
Several options:
Make all of it immutable. Initialize it in a static block and store it in a static variable (I'm not really a big fan of statics - it would be cleaner and more flexible use a dependency injection framework like Guice and inject it - then the decision about whether it's a singleton or not is made at startup time).
If doubtfulContext is not shared, and is the only thing which is stateful - then it is stateful, but any future caller will get a new instance of it, then your method is fine. If the doubtfulContext will be passed between threads later, you may need to make that thread-safe independently
If you want to optimize by, say, only reading the same file once and sharing an object that represents the file, then you will need some kind of thread-safe cache
Related
I have a data structure that I occasionally wish to modify, and occasionally wish to replace outright. At the moment, I'm storing this in an AtomicReference, and using synchrnonized blocks (synchronized on the AtomicReference itself, not its stored value) when I need to modify it, rather than replace it.
So something like:
public void foo(AtomicReference reference){
synchronized(reference){
reference.get()
.performSomeModification();
}
}
Notice that the modifying call is a member of the wrapped value, not the atomic reference, and is not guaranteed to have any thread safety of its own.
Is this safe? Findbugs (a freeware code reviewing tool) had this to say about it, so now I'm worried there's something happening under the hood, where it may be prematurely releasing the lock or something. I've also seen documentation referencing AtomicReference as specifically for immutable things.
Is this safe? If it isn't I could create my own Reference-storing class that I would be more certain about the behavior of, but I don't want to jump to conclusions.
From the linked documentation:
For example, synchronizing on an AtomicBoolean will not prevent other threads from modifying the AtomicBoolean.
It can't prevent other threads from modifying the AtomicBoolean because it can't force other threads to synchronize on the AtomicBoolean.
If I understand your question correctly, your intention is to synchronize calls to performSomeModification(). The code you've written will achieve that, if and only if every call to performSomeModification() is synchronized on the same object. As in the example from the docs, the basic problem is the enforceability of that requirement. You can't force other callers to synchronize on the AtomicReference. You or some other developer who comes after you could easily call performSomeModification() without external synchronization.
You should make it hard to use your API incorrectly. Since AtomicReference is a generic type (AtomicReference<V>), you can enforce the synchronization in a variety of ways, depending on what V is:
If V is an interface, you could easily wrap the instance in a synchronized wrapper.
If V is a class that you can modify, you could synchronize performSomeModification(), or create a subclass in which it is synchronized. (Possibly an anonymous subclass produced by a factory method.)
If V is a class that you cannot modify, it may be difficult to wrap. In that case, you could encapsulate the AtomicReference in a class that you do control, and have that class perform the required synchronization.
Are Mutable Atomic References a Bad Idea?
Definitely not! AtomicReference is designed to provide thread-safe, atomic updates of the underlying reference. In fact, the Javadoc description of AtomicReference is:
An object reference that may be updated atomically.
So they most definitely are designed to be mutated!
Is this safe?
It depends on what you mean by "safe", and what the rest of your code is doing. There's nothing inherently unsafe about your snippet of code in isolation. It's perfectly valid, though perhaps a bit unusual, to synchronize on an AtomicReference. As a developer unfamiliar with this code, I would see the synchronization on reference and assume that it means that the underlying object may be replaced at any time, and you want to make sure your code is always operating on the "newest" reference.
The standard best practices for synchronization apply, and violating them could result in unsafe behavior. For example, since you say performSomeModification() is not thread-safe, it would be unsafe if you accessed the underlying object somewhere else without synchronizing on reference.
public void bar(AtomicReference reference) {
// no synchronization: performSomeModification could be called on the object
// at the same time another thread is executing foo()
reference.get().performSomeModification();
}
If could also be "unsafe" if your application requires that only one instance of the underlying object be operated on at any one time, and you haven't synchronized on the reference when .set()ing it:
public void makeNewFoo(AtomicReference reference) {
// no synchronication on "reference", so it may be updated by another thread
// while foo() is executing performSomeModification() on the "old" reference
SomeObject foo = new SomeObject();
reference.set(foo);
}
If you need to synchronize on the AtomicReference, do so, it's perfectly safe. But I would highly recommend adding a few code comments about why you're doing it.
Is there any general way or rules exits by which we can ensure the thread safety of static methods specifically used in various Utility classes of any applications. Here I want to specifically point out the thread safety of Web Applications.
It is well know that static methods with Immutable Objects as parameters are thread safe and Mutable Objects are not.
If I have a utility method for some manipulation of java.util.Date and that method accepts an instance of java.util.Date, then this method would not be thread safe. Then how to make it thread safe without changing the way of parameter passing?
public class DateUtils {
public static Date getNormalizeDate(Date date) {
// some operations
}
}
Also is the class javax.faces.context.FacesContext mutable? Is it thread safe to pass an instance of this class to such static utility method?
This list of classes, instances of which can be or cannot be passed as parameters, could be long; so what points should we keep in mind while writing codes of such utility classes?
It is well known that static methods with immutable objects as parameters are thread safe and mutable objects are not.
I would contest this. Arguments passed to a method are stored on a stack, which is a per-thread idiom.
If your parameter is a mutable object such as a Date then you need to ensure other threads are not modifying it at the same time elsewhere. But that's a different matter unrelated to the thread-safety of your method.
The method you posted is thread-safe. It maintains no state and operates only on its arguments.
I would strongly recommend you read Java Concurrency in Practice, or a similar book dedicated to thread safety in Java. It's a complex subject that cannot be addressed appropriately through a few StackOverflow answers.
Since your class does not hold any member variables, your method is stateless (it only uses local variables and the argument) and therefore is thread safe.
The code that calls it might not be thread safe but that's another discussion. For example, Date not being thread safe, if the calling code reads a Date that has been written by another thread, you must use proper synchronization in the Date writing and reading code.
Given the structure of the JVM, local variables, method parameters, and return values are inherently "thread-safe." But instance variables and class variables will only be thread-safe if you design your class appropriately. more here
I see a lot of answers but none really pointing out the reason.
So this can be thought like this,
Whenever a thread is created, it is created with its own stack (I guess the size of the stack at the time of creation is ~2MB). So any execution that happens actually happens within the context of this thread stack.
Any variable that is created lives in the heap but it's reference lives in the stack with the exceptions being static variables which do not live in the thread stack.
Any function call you make is actually pushed onto the thread stack, be it static or non-static. Since the complete method was pushed onto the stack, any variable creation that takes place lives within the stack (again exceptions being static variables) and only accessible to one thread.
So all the methods are thread safe until they change the state of some static variable.
I would recommend creating a copy of that (mutable) object as soon as the method starts and use the copy instead of original parameter.
Something like this
public static Date getNormalizeDate(Date date) {
Date input = new Date(date.getTime());
// ...
}
Here's how I think of it: imagine a CampSite (that's a static method). As a camper, I can bring in a bunch of objects in my rucksack (that's arguments passed in on the stack). The CampSite provides me with a place to put my tent and my campstove, etc, but if the only thing the CampSite does is allow me to modify my own objects then it's threadsafe. The CampSite can even create things out of thin air (FirePit firepit = new FirePit();), which also get created on the stack.
At any time I can disappear with all my objects in my ruckstack and one of any other campers can appear, doing exactly what they were doing the last time they disappeared. Different threads in this CampSite will not have access to objects on the stack created CampSite in other threads.
Say there's only one campStove (a single object of CampStove, not separate instantiations). If by some stretch of the imagination I am sharing a CampStove object then there are multi-threading considerations. I don't want to turn on my campStove, disappear and then reappear after some other camper has turned it off - I would forever be checking if my hot dog was done and it never would be. You would have to put some synchronization somewhere... in the CampStove class, in the method that was calling the CampSite, or in the CampSite itself... but like Duncan Jones says, "that's a different matter".
Note that even if we were camping in separate instantiations of non-static CampSite objects, sharing a campStove would have the same multi-threading considerations.
We will take some examples to see if static method is Thread-Safe or not.
Example 1:
public static String concat (String st1, String str2) {
return str1 + str2
}
Now above method is Thread Safe.
Now we will see another example which is not thread-safe.
Example 2:
public static void concat(StringBuilder result, StringBuilder sb, StringBuilder sb1) {
result.append(sb);
result.append(sb1);
}
If you see both methods are very very primitive but still one is thread safe and other one is not. Why? What difference both having?
Are static methods in utilities prone to non thread-safe? Lot’s of questions right?
Now every thing depends on how you implement method & which type of objects you are using in your method. Are you using thread safe objects? Are these objects / classes are mutable?
If you see in Example 1 arguments of concat method are of type String which are immutable and passed by value so this method is completely thread-safe.
Now in Example 2 arguments are of StringBuilder type which are mutable so other thread can change value of StringBuilder which makes this method is potentially non thread-safe.
Again this is not completely true. If you are calling this utility method with local variables then you never having any problem related to thread-safety. Because each thread uses it’s own copy for local variables so you never run into any thread safety issues. But that is beyond scope of above static method. It’s depend on calling function / program.
Now static methods in utility class are kind of normal practice. So how we can avoid it? If you see Example 2 I am modifying 1st parameter. Now if you want to make this method really thread safe then one simple thing you can do. Either use non-mutable variables / objects or do not change / modify any method parameters.
In Example 2 we already used StringBuilder which is mutable so you can change implementation to make static method thread safe as follows:
public static String concat1(StringBuilder sb, StringBuilder sb1) {
StringBuilder result = new StringBuilder();
result.append(sb);
result.append(sb1);
return result.toString();
}
Again going to basics always remember if you are using immutable objects & local variables then you are miles away from thread safety issues.
From the arcticle(https://nikhilsidhaye.wordpress.com/2016/07/29/is-static-method-in-util-class-threadsafe/)Thank you Nikhil Sidhaye for this simple article
Lets say for example, a thread is creating and populating the reference variable of an immutable class by creating its object and another thread kicks in before the first one completes and creates another object of the immutable class, won't the immutable class usage be thread unsafe?
Creating an immutable object also means that all fields has to be marked as final.
it may be necessary to ensure correct behavior if a reference to
a newly created instance is passed from one thread to another without
synchronization
Are they trying to say that the other thread may re-point the reference variable to some other object of the immutable class and that way the threads will be pointing to different objects leaving the state inconsistent?
Actually immutable objects are always thread-safe, but its references may not be.
Confused?? you shouldn't be:-
Going back to basic:
Thread-safe simply means that two or more threads must work in coordination on the shared resource or object. They shouldn't over-ride the changes done by any other thread.
Now String is an immutable class, whenever a thread tries to change it, it simply end up creating a new object. So simply even the same thread can't make any changes to the original object & talking about the other thread would be like going to Sun but the catch here is that generally we use the same old reference to point that newly created object.
When we do code, we evaluate any change in object with the reference only.
Statement 1:
String str = "123"; // initially string shared to two threads
Statement 2:
str = str+"FirstThread"; // to be executed by thread one
Statement 3:
str=str+"SecondThread"; // to be executed by thread two
Now since there is no synchronize, volatile or final keywords to tell compiler to skip using its intelligence for optimization (any reordering or caching things), this code can be run in following manner.
Load Statement2, so str = "123"+"FirstThread"
Load Statement3, so str = "123"+"SecondThread"
Store Statement3, so str = "123SecondThread"
Store Statement2, so str = "123FirstThread"
and finally the value in reference str="123FirstThread" and for sometime if we assume that luckily our GC thread is sleeping, that our immutable objects still exist untouched in our string pool.
So, Immutable objects are always thread-safe, but their references may not be. To make their references thread-safe, we may need to access them from synchronized blocks/methods.
In addition to other answers posted already, immutable objects once created, they cannot be modified further. Hence they are essentially read-only.
And as we all know, read-only things are always thread-safe. Even in databases, multiple queries can read same rows simultaneously, but if you want to modify something, you need exclusive lock for that.
Immutable objects are thread safe, but why?
An immutable object is an object that is no longer modified once it has been constructed. If in addition, the immutable object is only made accessible to other thread after it has been constructed, and this is done using proper synchronization, all threads will see the same valid state of the object.
If one thread is creating populating the reference variable of the immutable class by creating its object and at the second time the other thread kicks in before the first thread completes and creates another object of the immutable class, won't the immutable class usage be thread unsafe?
No. What makes you think so? An object's thread safety is completely unaffected by what you do to other objects of the same class.
Are they trying to say that the other thread may re-point the reference variable to some other object of the immutable class and that way the threads will be pointing to different objects leaving the state inconsistent?
They are trying to say that whenever you pass something from one thread to another, even if it is just a reference to an immutable object, you need to synchronize the threads. (For instance, if you pass the reference from one thread to another by storing it in an object or a static field, that object or field is accessed by several threads, and must be thread-safe)
Thread safety is data sharing safety, And because in your code you make decisions based on the data your objects hold, the integrity and deterministic behaviour of it is vital. i.e
Imagine we have a shared boolean instance variable across two threads that are about to execute a method with the following logic
If flag is false, then I print "false" and then I set the flag back to true.
If flag is true, then I print "true" and then I set the flag back to false.
If you run continuously in a single thread loop, you will have a deterministic output which will look like:
false - true - false - true - false - true - false ...
But, if you ran the same code with two threads, then, the output of your output is not deterministic anymore, the reason is that the thread A can wake up, read the flag, see that is false, but before it can do anything, thread B wakes up and reads the flag, which is also false!! So both will print false... And this is only one problematic scenario I can think of... As you can see, this is bad.
If you take out the updates of the equation the problem is gone, just because you are eliminating all the risks associated with data sync. that's why we say that immutable objects are thread safe.
It is important to note though, that immutable objects are not always the solution, you may have a case of data that you need to share among different threads, in this cases there are many techniques that go beyond the plain synchronization and that can make a whole lot of difference in the performance of your application, but this is a complete different subject.
Immutable objects are important to guarantee that the areas of the application that we are sure that don't need to be updated, are not updated, so we know for sure that we are not going to have multithreading issues
You probably might be interested in taking a look at a couple of books:
This is the most popular: http://www.amazon.co.uk/Java-Concurrency-Practice-Brian-Goetz/dp/0321349601/ref=sr_1_1?ie=UTF8&qid=1329352696&sr=8-1
But I personally prefer this one: http://www.amazon.co.uk/Concurrency-State-Models-Java-Programs/dp/0470093552/ref=sr_1_3?ie=UTF8&qid=1329352696&sr=8-3
Be aware that multithreading is probably the trickiest aspect of any application!
Immutability doesn't imply thread safety.In the sense, the reference to an immutable object can be altered, even after it is created.
//No setters provided
class ImmutableValue
{
private final int value = 0;
public ImmutableValue(int value)
{
this.value = value;
}
public int getValue()
{
return value;
}
}
public class ImmutableValueUser{
private ImmutableValue currentValue = null;//currentValue reference can be changed even after the referred underlying ImmutableValue object has been constructed.
public ImmutableValue getValue(){
return currentValue;
}
public void setValue(ImmutableValue newValue){
this.currentValue = newValue;
}
}
Two threads will not be creating the same object, so no problem there.
With regards to 'it may be necessary to ensure...', what they are saying is that if you DON'T make all fields final, you will have to ensure correct behavior yourself.
I am reading Java Concurrency in Practice and kind of confused with the thread confinement concept. The book says that
When an object is confined to a thread, such usage is automatically thread-safe even if the confined object itself is not
So when an object is confined to a thread, no other thread can have access to it? Is that what it means to be confined to a thread? How does one keep an object confined to a thread?
Edit:
But what if I still want to share the object with another thread? Let's say that after thread A finishes with object O, thread B wants to access O. In this case, can O still be confined to B after A is done with it?
Using a local variable is one example for sure but that just means you don't share your object with other thread (AT ALL). In case of JDBC Connection pool, doesn't it pass one connection from one thread to another once a thread is done with that connection (totally clueless about this because I never used JDBC).
So when an object is confined to a thread, no other thread can have access to it?
No, it's the other way around: if you ensure that no other thread has access to an object, then that object is said to be confined to a single thread.
There's no language- or JVM-level mechanism that confines an object to a single thread. You simply have to ensure that no reference to the object escapes to a place that could be accessed by another thread. There are tools that help avoid leaking references, such as the ThreadLocal class, but nothing that ensures that no reference is leaked anywhere.
For example: if the only reference to an object is from a local variable, then the object is definitely confined to a single thread, as other threads can never access local variables.
Similarly, if the only reference to an object is from another object that has already been proven to be confined to a single thread, then that first object is confined to the same thread.
Ad Edit: In practice you can have an object that's only accessed by a single thread at a time during its lifetime, but for which that single thread changes (a JDBC Connection object from a connection pool is a good example).
Proving that such an object is only ever accessed by a single thread is much harder than proving it for an object that's confined to a single thread during its entire life, however.
And in my opinion those objects are never really "confined to a single thread" (which would imply a strong guarantee), but could be said to "be used by a single thread at a time only".
The most obvious example is use of thread local storage. See the example below:
class SomeClass {
// This map needs to be thread-safe
private static final Map<Thread,UnsafeStuff> map = new ConcurrentHashMap<>();
void calledByMultipleThreads(){
UnsafeStuff mystuff = map.get(Thread.currentThread());
if (mystuff == null){
map.put(Thread.currentThread(),new UnsafeStuff());
return;
}else{
mystuff.modifySomeStuff();
}
}
}
The UnsafeStuff objects itself "could be shared" with other threads in the sense that if you'd pass some other thread instead of Thread.currentThread() at runtime to the map's get method, you'd get objects belonging to other threads. But you are choosing not to. This is "usage that is confined to a thread". In other words, the runtime conditions are such that the objects is in effect never shared between different threads.
On the other hand, in the example below the object is automatically confined to a thread, and so to say, the "object itself" is confined to the thread. This is in the sense that it is impossible to obtain reference from other threads no matter what the runtime condition is:
class SomeClass {
void calledByMultipleThreads(){
UnsafeStuff mystuff = new UnsafeStuff();
mystuff.modifySomeStuff();
System.out.println(mystuff.toString());
}
}
Here, the UnsafeStuff is allocated within the method and goes out of scope when the method returns.. In other words, the Java spec is ensuring statically that the object is always confined to one thread. So, it is not the runtime condition or the way you use it that is ensuring the confinement, but more the Java spec.
In fact, modern JVM sometimes allocate such objects on stack, unlike the first example (haven't personally checked this, but I don't think at least current JVMs do).
Yet in other words, in the fist example the JVM can't be sure if the object is confined within a thread by just looking inside of calledByMultipleThreads() (who knows what other methods are messing with SomeClass.map). In the latter example, it can.
Edit: But what if I still want to
share the object with another thread?
Let's say that after thread A finishes
with object O, thread B wants to
access O. In this case, can O still be
confined to B after A is done with it?
I don't think it is called "confined" in this case. When you do this, you are just ensuring that an object is not accessed concurrently. This is how EJB concurrency works. You still have to "safely publish" the shared object in question to the threads.
So when an object is confined to a thread, no other thread can have access to it?
That's what thread confinement means - the object can only EVER be accessed by one thread.
Is that what it means to be confined to a thread?
See above.
How does one keep an object confined to a thread?
The general principle is to not put the reference somewhere that would allow another thread to see it. It is a little bit complicated to enumerate a set of rules that will ensure this, but (for instance) if
you create a new object, and
you never assign the object's reference to an instance or class variable, and
you never call a method that does this for the reference,
then the object will be thread confined.
I guess that's what want to say. Like creating a object inside the run method and not passing the reference to any other instance.
Simple example:
public String s;
public void run() {
StringBuilder sb = new StringBuilder();
sb.append("Hello ").append("world");
s = sb.toString();
}
The StringBuilder instance is thread-safe because it is confined to the thread (that executes this run method)
One way is "stack confinement" in which the object is a local variable confined to the thread's stack, so no other thread can access it. In the method below, the list is a local variable and doesn't escape from the method. The list doesn't have to be threadsafe because it is confined to the executing thread's stack. No other thread can modify it.
public String foo(Item i, Item j){
List<Item> list = new ArrayList<Item>();
list.add(i);
list.add(j);
return list.toString();
}
Another way of confining an object to a thread is through the use of a ThreadLocal variable which allows each thread to have its own copy. In the example below, each thread will have its own DateFormat object and so you don't need to worry about the fact that DateFormat is not thread-safe because it won't be accessed by multiple threads.
private static final ThreadLocal<DateFormat> df
= new ThreadLocal<DateFormat>(){
#Override
protected DateFormat initialValue() {
return new SimpleDateFormat("yyyyMMdd");
}
};
Further Reading
See: http://codeidol.com/java/java-concurrency/Sharing-Objects/Thread-Confinement/
A more formal means of maintaining
thread confinement is ThreadLocal,
which allows you to associate a
per-thread value with a value-holding
object. Thread-Local provides get and
set accessormethods that maintain a
separate copy of the value for each
thread that uses it, so a get returns
the most recent value passed to set
from the currently executing thread.
It holds a copy of object per one thread, thread A can't access copy of thread B and broke it's invariants if you will do it specially (for example, assign ThreadLocal value to static variable or expose it using other methods)
That's exactly what it means. The object itself is accessed by only one thread, and is thus thread-safe. ThreadLocal objects are a kind of objects that are bound to an only thread
I means that only code running in one thread accesses the object.
When this is the case, the object doesn't need to be "thread safe"
I was wondering if you have a static method that is not synchronised, but does not modify any static variables is it thread-safe? What about if the method creates local variables inside it? For example, is the following code thread-safe?
public static String[] makeStringArray( String a, String b ){
return new String[]{ a, b };
}
So if I have two threads calling ths method continously and concurrently, one with dogs (say "great dane" and "bull dog") and the other with cats (say "persian" and "siamese") will I ever get cats and dogs in the same array? Or will the cats and dogs never be inside the same invocation of the method at the same time?
This method is 100% thread safe, it would be even if it wasn't static. The problem with thread-safety arises when you need to share data between threads - you must take care of atomicity, visibility, etc.
This method only operates on parameters, which reside on stack and references to immutable objects on heap. Stack is inherently local to the thread, so no sharing of data occurs, ever.
Immutable objects (String in this case) are also thread-safe because once created they can't be changed and all threads see the same value. On the other hand if the method was accepting (mutable) Date you could have had a problem. Two threads can simultaneously modify that same object instance, causing race conditions and visibility problems.
A method can only be thread-unsafe when it changes some shared state. Whether it's static or not is irrelevant.
The function is perfectly thread safe.
If you think about it... assume what would happen if this were different. Every usual function would have threading problems if not synchronized, so all API functions in the JDK would have to be synchronized, because they could potentially be called by multiple threads. And since most time the app is using some API, multithreaded apps would effectively be impossible.
This is too ridiculous to think about it, so just for you: Methods are not threadsafe if there is a clear reason why there could be problems. Try to always think about what if there were multiple threads in my function, and what if you had a step-debugger and would one step after another advance the first... then the second thread... maybe the second again... would there be problems? If you find one, its not thread safe.
Please be also aware, that most of the Java 1.5 Collection classes are not threadsafe, except those where stated, like ConcurrentHashMap.
And if you really want to dive into this, have a close look at the volatile keyword and ALL its side effects. Have a look at the Semaphore() and Lock() class, and their friends in java.util.Concurrent. Read all the API docs around the classes. It is worth to learn and satisfying, too.
Sorry for this overly elaborate answer.
Use the static keyword with synchronized static methods to modify static data shared among threads. With the static keyword all the threads created will contend for a single version of the method.
Use the volatile keyword along with synchronized instance methods will guarantee that each thread has its own copy of the shared data and no read/ writes will leak out between threads.
String objects being immutable is another reason for thread-safe scenario above. Instead if mutable objects are used (say makeMutableArray..) then surely thread-safety will break.
Since the complete method was pushed onto the stack, any variable creation that takes place lives within the stack (again exceptions being static variables) and only accessible to one thread. So all the methods are thread safe until they change the state of some static variable.
See also:
Is static method is thread safe in Java?