Thread Safety in Java – Thread safety in java is the process to make our program safe to use in multithreaded environment, there are different ways through which we can make our program thread safe.

Synchronization is the easiest and most widely used tool for thread safety in java. Use of Atomic Wrapper classes from java.util.concurrent.atomic package. For example AtomicInteger Use of locks from java.util.concurrent.locks package. Using thread safe collection classes, check this post for usage of ConcurrentHashMap for thread safety. Using volatile keyword with variables to make every thread read the data from memory, not read from thread cache.

What does thread safety mean?

Thread Safety Multithreaded Programming Guide Thread safety is the avoidance of data races-situations in which data are set to either correct or incorrect values, depending upon the order in which multiple threads access and modify the data. When no sharing is intended, give each thread a private copy of the data.

Unsafe Thread safe – Serializable Thread safe – MT-Safe

An unsafe procedure can be made thread safe and serializable by surrounding it with statements to lock and unlock a mutex. shows three simplified implementations of fputs(), initially thread unsafe. Next is a serializable version of this routine with a single mutex protecting the procedure from concurrent execution problems.

Actually, this is stronger synchronization than is usually necessary. When two threads are sending output to different files using fputs(), one need not wait for the other-the threads need synchronization only when they are sharing an output file. The last version is MT-safe. It uses one lock for each file, allowing two threads to print to different files at the same time.

So, a routine is MT-safe when it is thread safe and its execution does not negatively affect performance.

What do you understand by thread safety Why is it required?

Levels of thread safety – Software libraries can provide certain thread-safety guarantees. For example, concurrent reads might be guaranteed to be thread-safe, but concurrent writes might not be. Whether a program using such a library is thread-safe depends on whether it uses the library in a manner consistent with those guarantees.

Thread safe : Implementation is guaranteed to be free of race conditions when accessed by multiple threads simultaneously. Conditionally safe : Different threads can access different objects simultaneously, and access to shared data is protected from race conditions. Not thread safe : Data structures should not be accessed simultaneously by different threads.

Thread safety guarantees usually also include design steps to prevent or limit the risk of different forms of deadlocks, as well as optimizations to maximize concurrent performance. However, deadlock-free guarantees cannot always be given, since deadlocks can be caused by callbacks and violation of architectural layering independent of the library itself.

What are the examples of thread-safe in Java?

What is thread safety ? thread-safety or thread-safe code in Java refers to code that can safely be utilized or shared in concurrent or multi-threading environment and they will behave as expected. Thread-safety is one of the risks introduced by using threads in Java and I have seen java programmers and developers struggling to write thread-safe code or just understanding what is thread-safe code and what is not? Before you learn how to write a thread-safe code you need to understand what is thread-safety and there is no better way to that than looking at a non-thread-safe code.

So, let’s see an example of the potential, not thread-safe code, and learn how to fix that. Here is an example of a non-thread-safe code, look at the code, and find out why this code is not thread-safe ? Example 1 public class NumberCounter public int getCountAtomically() } The above example is not thread-safe because ++ (the increment operator) is not an atomic operation and can be broken down into reading, update, and write operations.

If multiple thread call getCount() approximately same time each of these three operations may coincide or overlap with each other for example while thread 1 is updating value, thread 2 reads and still gets old value, which eventually let thread 2 override thread 1 increment and one count is lost because multiple threads called it concurrently.

Example 2 public class BasicObservableClass private Set mObservers; public void registerObserver(Observer observer) if (mObservers == null) mObservers.add(observer); } public void unregisterObserver(Observer observer) } private void notifyObservers() for (Observer observer : mObservers) } } In the most general case of multi-threaded environment we shall assume that all methods will be called on random threads at random times.

The implementation we used for a single-threaded environment is not safe anymore. It is not hard to imagine a flow when two distinct threads attempt to register new Observers at the same time, and end up leaving our system screwed up. One such flow could be (there are many other flows which could break non-thread-safe implementation):

Thread A invokes registerObserver(Observer) Thread A executes mObservers == null check and proceeds to instantiation of a new setbefore Thread A got a chance to create a new set, OS suspended it and resumed execution of Thread BThread B executes steps 1 and 2 abovesince Thread A hasn’t instantiated the set yet, Thread B instantiates a new set and stores a reference to it in mObservers Thread B adds an observer to the newly created setat some point OS resumes execution of Thread A (which was suspended right before instantiation of a new set)Thread A instantiates a new set and overrides the reference in mObservers Thread A adds an observer to the newly created set

Despite the fact that both calls to registerObserver(Observer) completed successfully, the end result is that only one observer will be notified when notifyObservers() called. It is important to understand that any of the observers could end up being “ignored”, or both observers could be registered successfully – the outcome depends on the scheduling of threads by OS which we can’t control.

This non-determinism is what makes multi-threading bugs very hard to track and resolve. There are multiple ways to make this code thread-safe in Java: 1) Use the synchronized keyword in Java and lock the getCount() method so that only one thread can execute it at a time which removes the possibility of coinciding or interleaving.2) use Atomic Integer, which makes this ++ operation atomic and since atomic operations are thread-safe and saves the cost of external synchronization.

Here is a thread-safe version of NumberCounter class in Java: public class NumberCounter public int getCountAtomically() } Here are some points worth remembering to write thread-safe code in Java, this knowledge also helps you to avoid some serious concurrency issues in Java-like race condition or deadlock in Java: 1) Immutable objects are by default thread-safe because their state can not be modified once created.

How to make a Java class thread-safe?

The general solution is to use keyword synchronized on the methods or even on your any private logically used object for this purpose. This keyword just locks the object and you are guaranteed that only one thread will be available to access this method at any given time.

How stack is thread-safe in Java?

Implement a multi-threaded stack, with freedom to use existing implementations of stack. On top of being thread-safe, it must block (not busy-wait) a pushing thread when stack is full and a popping thread when stack is empty.

Which variables are thread-safe in Java?

4. FAQs – Why are local variables thread-safe in Java? Local variables in Java are thread-safe because they are stored on the stack memory, which is private to each thread. This means that each thread has its own copy of the local variables, which eliminates the possibility of concurrent access and modification by other threads.

Are all variables thread-safe in Java? No, not all variables in Java are thread-safe. Variables that are shared between threads, such as instance variables and static variables, are not thread-safe by default and require synchronization to ensure thread safety. How can I ensure thread safety when using shared resources in Java? To ensure thread safety when using shared resources in Java, you can use synchronization mechanisms such as synchronized methods, synchronized blocks, or the java.util.concurrent package.

You can also use thread-safe collections such as ConcurrentHashMap or synchronized collections. Can thread safety be achieved in Java without synchronization? Yes, thread safety can be achieved in Java without synchronization by using thread-local variables or immutable objects.

Which class is not thread-safe in Java?

Java Examples in a Nutshell, 3rd Edition Get full access to Java Examples in a Nutshell, 3rd Edition and 60K+ other titles, with a free 10-day trial of O’Reilly. There are also live events, courses curated by job role, and more. When designing a class that may be used for concurrent programming—that is, a class whose instances may be used by more than one thread at a time—it is imperative that you make sure the class is ” thread-safe.” Consider the IntList class of,

This class is not thread safe. Imagine what could happen if one thread called clear( ) while another thread was calling add( ), If the clear( ) method sets the list size to 0 after add( ) has read the list size but before it has stored the incremented list size back into the size field of the IntList, it may appear as if the call to clear( ) never happened! In general, a thread-safe class ensures that no thread can ever observe its instances in an inconsistent state.

There are several approaches to thread safety. A particularly simple one is to design immutable classes: if the state of an object can never change, then no thread can ever observe the object in an inconsistent state. Some classes, such as IntList, must be mutable, however.

• To make these classes thread-safe, you must prevent concurrent access to the internal state of an instance by more than one thread.
• Because Java was designed with threads in mind, the language provides the synchronized modifier, which does just that.
• When an instance method is declared synchronized, a thread must obtain a lock on the instance before it calls the method.

If the lock is already held by another thread, the thread blocks until it can obtain the lock it needs. This ensures that only one thread may call any of the synchronized methods of the instance at a time. is a simplified version of the IntList class of whose methods have been declared synchronized,

This prevents two threads from calling the add( ) method at the same time, and also prevents a thread from calling clear( ) while another thread is calling add( ), The synchronized keyword can also be applied to arbitrary blocks of code within a method, simply by specifying the object to be locked before the code is executed.

The ThreadSafeIntList( ) copy constructor uses this technique to synchronize access to the internal state of the object it is copying. Note that it is not good design to declare every method of every class synchronized, Calling a synchronized method is substantially slower than calling a nonsynchronized one because of the overhead of object locking.

The java.util.Vector class that shipped with the original version of Java has synchronized methods to guarantee thread safety. But most applications do not require thread safety, and Java 1.2 provided the more efficient unsynchronized alternative java.util.ArrayList, Example 4-2. ThreadSafeIntList.java package je3.thread; /** * A growable array of int values, suitable for use with multiple threads.

**/ public class ThreadSafeIntList // This constructor returns a copy of an existing ThreadSafeIntList. // Note that it synchronizes its access to the original list. public ThreadSafeIntList(ThreadSafeIntList original) } // Return the number of ints stored in the list public synchronized int size( ) // Return the int stored at the specified index public synchronized int get(int index) // Append a new value to the list, reallocating if necessary public synchronized void add(int value) // Remove all elements from the list public synchronized void clear( ) // Copy the contents of the list into a new array and return that array public synchronized int toArray( ) // Reallocate the data array to enlarge or shrink it.

• // Not synchronized, because it is always called from synchronized methods.
• Protected void setCapacity(int n) } Get Java Examples in a Nutshell, 3rd Edition now with the O’Reilly learning platform.
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What is difference between thread-safe and non thread-safe?

What is thread safe or non-thread safe in PHP ?

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Thread-safe: It is used to ensure that when the shared data structure which is manipulated by different threads are prevented from entering the race condition. Thread-safety is recommended when the web server run multiple threads of execution simultaneously for different requests.

In Thread Safety binary can work in a multi-threaded web server context. Thread Safety works by creating a local storage copy in each thread so that the data will not collide with another thread. For example: Non-thread-safe: It does not check the safety of the threads which makes it faster to run but at the same time, it becomes more unstable and crashes very frequently.

It refers to a single thread only builds. In non-thread safe version binaries widespread use in the case of interaction with a web server through the FastCGI protocol, by not utilizing multi-threading. For example:

• Apache + FastCGI
• IIS + FastCGI

So it depends on the way that you want to use PHP. AFAIR running PHP with the fastCGI is the preferable way. If you are unknown which version of PHP is installed in your system then there is an easy way to know that. Check the version of installed PHP Thread safe or Non Thread Safe: Open a phpinfo() and search for the line Thread safety for a thread-safe build you should find enable.

• On Windows: php -i|findstr “Thread”
• On *nix: php -i|grep Thread
• In the both cases will display any one Thread Safety => enabled //or Thread Safety => disabled

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Is thread-safe and immutable?

To put it simply, a class instance is immutable when its internal state can’t be modified after it has been constructed. A MessageService object is effectively immutable since its state can’t change after its construction. So, it’s thread-safe.

How static is thread-safe?

Thread Safety – Static variables are not thread safe. Instance variables do not require thread synchronization unless shared among threads. But, static variables are always shared by all the threads in the process. Hence, access to static variable is not thread safe.

How to know if Java class is thread-safe?

How to know if a Java SE class or method is thread safe?

For a standard Java SE class, the best way to know whether or not the class is thread-safe is to carefully read its documentation. Always read both the class documentation and the method documentation. If either say it’s not synchronized or not thread-safe, you know it’s not thread-safe. Therefore, the DateFormat class is not thread safe. The documentation specifically says: Date formats are not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally. Declaring a field private does not make your implementation thread-safe. private merely says that outside classes can’t see that field. Let’s look at your method: for (int i=0;i<10;i++) new Thread(new Runnable() }); The Runnable objects that you create are anonymous classes. Anonymous classes are inner classes, which have access to private fields of their surrounding class. If it wasn't so, your program would not compile - they could not access the df field. But they can. So in fact you are having 10 threads that are all accessing your one DateFormat object, referred to by df, Since we already know that DateFormat is not thread-safe, your program is not thread-safe. Furthermore, if two external threads have references to your object (I mean the object that has the df inside it. You didn't give the class declaration so I don't know what its name is). They have references to the same instance of your class. If both of them call format at the same time, both will be running DateFormat.format using the same private df, Thus, this is not going to be thread-safe. To be thread-safe, you need to synchronize on the object or use some other kind of lock (one lock for all the possible threads that access it), which is exactly what the documentation said to do. Another way is to have a completely local object, which is visible to only one thread. Not a field - a local variable, which has access to a uniquely created instance of DateFormat (so you have a new copy every time you call the method). Beware of anonymous classes, though! In your example, even if df was a local field to the format method, it would still not be thread-safe because all your threads would be accessing the same copy.

: How to know if a Java SE class or method is thread safe?

What is deadlock in Java?

Deadlock in java is a programming situation where two or more threads are blocked forever. Java deadlock situation arises with at least two threads and two or more resources. Here I have written a simple program that will cause java deadlock scenario and then we will see how to analyze it.

How to test thread-safe in Java?

Learn how to test if a class is thread-safe in Java. Tests for thread safety differ from typical single-threaded tests. To test if a method is thread-safe we need to call the method in parallel from multiple threads. We need to do this for all potential thread interleavings.

1. And afterward, we need to check if the result is correct.
2. Those three requirements for our test lead to a special type of tests for thread safety which differ from typical single-threaded tests.
3. Since we want to test all thread interleavings our test must be repeatable and run automatically.
4. And since the methods run in parallel the potential result is a combination of different outcomes.

You may also like: What Does Thread-Safety Mean in Java? Let us look at an example to see how this looks in practice.

Is singleton thread-safe?

How to make a singleton thread-safe? – The short answer is a singleton can be made thread-safe by instantiating the singleton class inside a static inner class or static initializer block,

Why singleton beans are not thread-safe?

Making the Bean Thread-Safe – Going back to our previous point, singleton beans are not inherently thread safe, This is because these beans run for the entire lifetime of the application and many HTTP(S) requests are made from different users at the same time. So how can we fix that?

How do I prove HashMap is not thread-safe?

Java Collection classes are heart of Java API. It is essential to use built-in java collections like HashMap, ArrayList or LinkedList for accessing, storing and processing data in java applications. For Example, we have extensively used HashMap to transfer data between two layers of our MVC framework. In all core java interviews, you will definitely face questions on HashMap Internals like,

• what is the use of hashCode() and equals() method?
• How put() and get() method works in HashMap ?

or you may face question like How is HashSet implemented internally in Java? As a follow up question to above questions, interviewer may ask you : is HashMap a thread-safe class? Well, HashMap is not thread-safe, If multiple threads are accessing the same HashMap object and try to modify the structure of the HashMap (using put() or remove() method), it may cause an inconsistency in the state of HashMap,

To use HashMap in multithreaded environment, you must write your relevant code inside synchronized block or use any external Lock implementation. But in that case there are high chances of errors and deadlock situations, if proper care has not been taken. In short, it is not advisable to use HashMap in multithreaded environment.

Instead use any of the similar thread-safe collections like Hashtable, Collections.SynchronizedMap or ConcurrentHashMap, Though all of them are thread-safe, ConcurrentHashMap provides better performance than remaining two. lets understand them one by one.

What is thread safety vs synchronization?

Thread Safety in Java – Thread safety in java is the process to make our program safe to use in multithreaded environment, there are different ways through which we can make our program thread safe.

Synchronization is the easiest and most widely used tool for thread safety in java. Use of Atomic Wrapper classes from java.util.concurrent.atomic package. For example AtomicInteger Use of locks from java.util.concurrent.locks package. Using thread safe collection classes, check this post for usage of ConcurrentHashMap for thread safety. Using volatile keyword with variables to make every thread read the data from memory, not read from thread cache.

What is meant by thread safety in swing?

Why Swing Components are not thread-safe –

One of the main reason for Java Swing is not thread-safe is to simplify the task of extending its components.Another reason for the Java Swing is not thread-safe due to the overhead involved in obtaining and releasing locks and restoring the state,Some of the Java Swing component methods will support multi-threaded access like repaint(), revalidate(), and invalidate() methods of JComponent class.

What is thread safety in IOS?

Thread Safe – If any object is not allowed to modify by more than one thread at the same time. Generally, immutable objects are thread-safe.

Jack Gloop