Why double checked locking is broken?

Why double checked locking is broken?

DoubleCheckedLocking, a common CeePlusPlus and JavaLanguage Idiom for lazy initialization in multi-thread programs, commonly used for implementing singletons, turns out to not work reliably because of issues with optimizing compilers and shared memory multiprocessors.

What is double checked locking in Java?

In double-checked locking, code checks for an existing instance of Singleton class twice with and without locking to make sure that only one instance of singleton gets created.

Is double checked locking thread safe?

In multi-threaded environments, initialization is usually not thread safe, so locking is required to protect the critical section. ... Since only the first access requires locking, double-checked locking is used to avoid locking overhead of subsequent accesses.

What is the use of double checked locking in Singleton?

The double checked pattern is used to avoid obtaining the lock every time the code is executed. If the call are not happening together then the first condition will fail and the code execution will not execute the locking thus saving resources.

What is double locking?

: to lock with two bolts or by two turns of the key : fasten doubly.

How do you break a singleton?

Suppose you serialize an object of a singleton class. Then if you de-serialize that object it will create a new instance and hence break the singleton pattern. As you can see, hashCode of both instances is different, hence there are 2 objects of a singleton class.

Why volatile is used in Singleton?

The volatile prevents memory writes from being re-ordered, making it impossible for other threads to read uninitialized fields of your singleton through the singleton's pointer. ... Thread A now writes the reference to the newly created singleton into uniqueInstance , and gets ready to release its lock.

Is volatile thread safe?

Volatile keyword is used to modify the value of a variable by different threads. It is also used to make classes thread safe. It means that multiple threads can use a method and instance of the classes at the same time without any problem. The volatile keyword can be used either with primitive type or objects.

Is double locking must to create thread safe Singleton class?

By the way this is not the best way to create thread-safe Singleton, you can use Enum as Singleton, which provides inbuilt thread-safety during instance creation. Another way is to use static holder pattern. That's all about double checked locking of Singleton class in Java.

What is the difference between volatile and synchronized?

So where volatile only synchronizes the value of one variable between thread memory and "main" memory, synchronized synchronizes the value of all variables between thread memory and "main" memory, and locks and releases a monitor to boot. Clearly synchronized is likely to have more overhead than volatile.

Is volatile synchronized?

Effectively, a variable declared volatile must have its data synchronized across all threads, so that whenever you access or update the variable in any thread, all other threads immediately see the same value. Generally, volatile variables have a higher access and update overhead than "plain" variables.

Is AtomicInteger thread safe?

AtomicInteger lets us update integers in a thread-safe way. Use atomic methods like incrementAndGet or decrementAndGet for simple types of calculations. And use the methods get and compareAndSet for all other types of calculations.

What is the difference between volatile and atomic variables?

Volatile and Atomic are two different concepts. Volatile ensures, that a certain, expected (memory) state is true across different threads, while Atomics ensure that operation on variables are performed atomically.

Are atomic variables volatile?

1) Atomic variables in Java also provides the same memory semantics as a volatile variable, but with an added feature of making compound action atomic. 2) It provides a convenient method to perform atomic increment, decrement, CAS operations.

What is atomic keyword in Java?

Atomicity. Atomic operations are those operations that ALWAYS execute together. Either all of them execute together, or none of them executes. If an operation is atomic, then it cannot be partially complete, either it will be complete, or not start at all, but will not be incomplete.

What is the difference between transient and volatile variable in Java?

The volatile keyword flushes the changes directly to the main memory instead of the CPU cache. ... On the other hand, the transient keyword is used during serialization. Fields that are marked as transient can not be part of the serialization and deserialization.

What is volatile modifier?

The volatile modifier is used to let the JVM understand that a thread accessing the variable should always merge its own personal copy of the variable with the original in the memory. Accessing a volatile variable synchronizes all the cached copy of the variables in the main memory.

What is transient variable?

transient is a variables modifier used in serialization. At the time of serialization, if we don't want to save value of a particular variable in a file, then we use transient keyword. When JVM comes across transient keyword, it ignores original value of the variable and save default value of that variable data type.

What is transient keyword in Java?

The transient keyword in Java is used to avoid serialization. If any object of a data structure is defined as a transient , then it will not be serialized. Serialization is the process of converting an object into a byte stream.

What is difference between transient and static in serialization?

Static Variables: These variables are not serialized, So during deserialization static variable value will loaded from the class. ... transient Variables: transient variables are not serialized, so during deserialization those variables will be initialized with corresponding default values (ex: for objects null , int 0 ).

What is a transient?

transient, transitory, ephemeral, momentary, fugitive, fleeting, evanescent mean lasting or staying only a short time. transient applies to what is actually short in its duration or stay.

What is the use of transient keyword in Salesforce?

Use the transient keyword to declare instance variables that can't be saved, and shouldn't be transmitted as part of the view state for a Visualforce page.

What is pagination in salesforce?

Pagination is the process of taking a complete document or a large number of records in this example and breaking that document/large number of records into separate pages for viewing within the Salesforce. ... The Salesforce user can click the next tab and go through each new page display the next 10 records.

What is the view state in Salesforce?

View state holds the state/size of the visual force page that includes the components, field values, and controller state. This is important in the light that salesforce provides only a standard size of 135kb for any individual page.

What is static variable in Salesforce?

Static variables are variables that belong to an overall class, not a particular object of a class. Think of a static variable to be like a global variable – its value is shared across your entire org. Any particular object's properties or values are completely irrelevant when using static.

Why future method is static in Salesforce?

Future methods can be accessed from any class and not depend on any class instance hence it is declared as Static. ... Future method runs on it own thread, so it cannot return any values to previous instance.

What are three characteristics of static methods Salesforce?

Static methods, variables, and initialization code have these characteristics.

They're associated with a class.
They're allowed only in outer classes.
They're initialized only when a class is loaded.
They aren't transmitted as part of the view state for a Visualforce page.

What is the use of global keyword in Salesforce?

Global provides a hieger level of access than Public. Any global method or variable can be accessed by any apex code that has access to the class not only from the same application but from outside the application.

How do I fix heap size limit exceeding?

Build better apex scripts to manage heap limits

Look for heap size in debug logs. If heap size is approaching the limit, investigate it and refactor the code.
Use the 'Transient' keyword with variables. ...
Use Limit methods. ...
Reduce heap size during runtime by removing items from the collection as you iterate over it.
Use SOQL for loops.

What is standard set controller in Salesforce?

StandardSetController objects allow you to create list controllers similar to, or as extensions of, the pre-built Visualforce list controllers provided by Salesforce.

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