It’s also possible to achieve thread-safety using the set of atomic classes that Java provides, including AtomicInteger, AtomicLong, AtomicBoolean and AtomicReference. Atomic classes allow us to perform atomic operations, which are thread-safe, without using synchronization.

What is the alternative to synchronized method in Java?

Synchronized Block Limitations and Alternatives – Synchronized blocks in Java have several limitations. For instance, a synchronized block in Java only allows a single thread to enter at a time. However, what if two threads just wanted to read a shared value, and not update it? That might be safe to allow.

1. As alternative to a synchronized block you could guard the code with a Read / Write Lock which as more advanced locking semantics than a synchronized block.
2. Java actually comes with a built in ReadWriteLock class you can use.
3. What if you want to allow N threads to enter a synchronized block, and not just one? You could use a Semaphore to achieve that behaviour.

Java actually comes with a built-in Java Semaphore class you can use. Synchronized blocks do not guarantee in what order threads waiting to enter them are granted access to the synchronized block. What if you need to guarantee that threads trying to enter a synchronized block get access in the exact sequence they requested access to it? You need to implement Fairness yourself.

What is the alternative to synchronized?

Locking on an Object field : the premises of the lock API – Here is the first paragraphs of the the Lock interface javadoc (emphasis is mine) : Lock implementations provide more extensive locking operations than can be obtained using synchronized methods and statements,

They allow more flexible structuring, may have quite different properties, and may support multiple associated Condition objects. A lock is a tool for controlling access to a shared resource by multiple threads. Commonly, a lock provides exclusive access to a shared resource : only one thread at a time can acquire the lock and all access to the shared resource requires that the lock be acquired first.

However, some locks may allow concurrent access to a shared resource, such as the read lock of a ReadWriteLock, The use of synchronized methods or statements provides access to the implicit monitor lock associated with every object, but forces all lock acquisition and release to occur in a block-structured way: when multiple locks are acquired they must be released in the opposite order, and all locks must be released in the same lexical scope in which they were acquired.

While the scoping mechanism for synchronized methods and statements makes it much easier to program with monitor locks, and helps avoid many common programming errors involving locks, there are occasions where you need to work with locks in a more flexible way, For example, some algorithms for traversing concurrently accessed data structures require the use of « hand-over-hand » or « chain locking »: you acquire the lock of node A, then node B, then release A and acquire C, then release B and acquire D and so on.

Implementations of the Lock interface enable the use of such techniques by allowing a lock to be acquired and released in different scopes, and allowing multiple locks to be acquired and released in any order, With this increased flexibility comes additional responsibility.

The absence of block-structured locking removes the automatic release of locks that occurs with synchronized methods and statements. In bold, you can read the main features and constraints brought by the Lock API. To sum up main of these: more flexible, more lock features (scope, order), read and write lock flavors and as a consequence more responsibility for the developers.

We will go on by showing how to do the same thing with synchronized statements and ReentrantLock : the most basic lock implementation.

Is synchronized and thread-safe same?

Thread safe means: method becomes safe to be accessed by multiple threads without any problem at the same time. synchronized keyword is one of the way to achieve ‘thread safe’. But Remember:Actually while multiple threads tries to access synchronized method they follow the order so becomes safe to access.

Which is better synchronized block or method?

Points to Remember –

• Synchronized block is used to lock an object for any shared resource.
• Scope of synchronized block is smaller than the method.
• A Java synchronized block doesn’t allow more than one JVM, to provide access control to a shared resource.
• The system performance may degrade because of the slower working of synchronized keyword.
• Java synchronized block is more efficient than Java synchronized method.

Is ConcurrentHashMap thread-safe?

Java – import java.util.concurrent.ConcurrentHashMap; public class Main } Output Map size: 3 Value of A: 1 Map size: 2 Key points of ConcurrentHashMap:

• The underlined data structure for ConcurrentHashMap is Hashtable,
• ConcurrentHashMap class is thread-safe i.e. multiple threads can operate on a single object without any complications.
• At a time any number of threads are applicable for a read operation without locking the ConcurrentHashMap object which is not there in HashMap.
• In ConcurrentHashMap, the Object is divided into a number of segments according to the concurrency level.
• The default concurrency-level of ConcurrentHashMap is 16.
• In ConcurrentHashMap, at a time any number of threads can perform retrieval operation but for updated in the object, the thread must lock the particular segment in which the thread wants to operate. This type of locking mechanism is known as Segment locking or bucket locking, Hence at a time, 16 update operations can be performed by threads.
• Inserting null objects is not possible in ConcurrentHashMap as a key or value.

Declaration: public class ConcurrentHashMap extends AbstractMap implements ConcurrentMap, Serializable Here, K is the key Object type and V is the value Object type. The Hierarchy of ConcurrentHashMap It implements Serializable, ConcurrentMap, Map interfaces and extends AbstractMap class. Constructors of ConcurrentHashMap

• Concurrency-Level: It is the number of threads concurrently updating the map. The implementation performs internal sizing to try to accommodate this many threads.
• Load-Factor: It’s a threshold, used to control resizing.
• Initial Capacity: Accommodation of a certain number of elements initially provided by the implementation. if the capacity of this map is 10. It means that it can store 10 entries.

1. ConcurrentHashMap() : Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16). ConcurrentHashMap chm = new ConcurrentHashMap (); 2. ConcurrentHashMap(int initialCapacity) : Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16).

ConcurrentHashMap chm = new ConcurrentHashMap (int initialCapacity); 3. ConcurrentHashMap(int initialCapacity, float loadFactor) : Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16). ConcurrentHashMap chm = new ConcurrentHashMap (int initialCapacity, float loadFactor); 4.

ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) : Creates a new, empty map with the specified initial capacity, load factor, and concurrency level. ConcurrentHashMap chm = new ConcurrentHashMap (int initialCapacity, float loadFactor, int concurrencyLevel); 5.

How do I make sure my class is thread-safe?

Thread Safe Class means that every changes (getting/setting values) into your POJO class are made Thread Safely. It can be achieved by synchronization mechanism. The general solution is to use keyword synchronized on the methods or even on your any private logically used object for this purpose.

What is the alternative to synchronized in Java 8?

15. June 2016 To provide synchronized data cache access, I discuss three alternatives in Java 8: synchronized() blocks, ReadWriteLock and StampedLock (new in Java 8). I show code snippets and compare the performance impact on a real world application.

Consider the following use case: A data cache that holds key-value pairs and needs to be accessed by several threads concurrently. One option is to use a synchronized container like ConcurrentHashMap or Collections.synchronizedMap(map), Those have their own considerations, but will not be handled in this article.

In our use case, we want to store arbitrary objects into the cache and retrieve them by Integer keys in the range of 0.n. As memory usage and performance is critical in our application, we decided to use a good, old array instead of a more sophisticated container like a Map.

Memory visibility: Threads may see the array in different states (see explanation ).Race conditions: Writing at the same time may cause one thread’s change to be lost (see explanation )

Thus, we need to provide some form of synchronization. To fix the problem of memory visibility, Java’s volatile keyword seems to be the perfect fit. However, making an array volatile has not the desired effect because it makes accesssing the array variable atomic, but not accessing the arrays content,

1. In case the array’s payload is Integer or Long values, you might consider AtomicIntegerArray or AtomicLongArray,
2. But in our case, we want to support arbitrary values, i.e. Objects.
3. Traditionally, there are two ways in Java to do synchronization: synchronized() blocks and ReadWriteLock,
4. Java 8 provides another alternative called StampedLock,

There are propably more exotic ways, but I will focus on these three relatively easy to implement and well understood ways. For each approach, I will provide a short explanation and a code snippet for the cache’s read and write methods. synchronized is a Java keyword that can be used to restrict the execution of code blocks or methods to one thread at a time.

ReadWriteLock is an interface. If I say ReadWriteLock, I mean its only standard library implementation ReentrantReadWriteLock, The basic idea is to have two locks: one for write access and one for read access. While writing locks out everyone else (like synchronized), multiple threads may read concurrently.

StampedLock is a new addition in Java 8. It is similiar to ReadWriteLock in that it also has separate read and write locks. The methods used to aquire locks return a “stamp” (long value), that represents a lock state. I like to think of the stamp as the “version” of the data in terms of data visibility.

1. This makes a new locking strategy possible: the “optimistic read”.
2. An optimistic read means to aquire a stamp (but no actual lock), read without locking and afterwards validate the lock, i.e.
3. Check if it was ok to read without a lock.
4. If we were too optimistic and it turns out someone else wrote in the meantime, the stamp would be invalid.

In this case, we have no choice but to acquire a real read lock and read the value again. Like ReadWriteLock, StampedLock is efficient if there is more read than write access. It can save a lot overhead to not have to acquire and release locks for every read access.

All three alternatives are valid choices for our cache use case, because we expect more reads than writes. To find out which is best, I ran a benchmark with our application. The test machine is a Intel Core i7-5820K CPU which has 6 physical cores (12 logical cores with hyper threading).

1. Our application spawns 12 threads that access the cache concurrently.
2. The application is a “loader” that imports data from a database, makes calculations and stores the results into a database.
3. The cache is not under stress 100% of the time.
4. However it is vital enough to show a significant impact on the application’s overall runtime.

As benchmark I executed our application with reduced data. To get a good average, I ran each locking strategy three times. Here are the results: In our use case, StampedLock provides the best performance. While 15% difference to synchronized and 24% difference to ReadWriteLock may not seem much, it is relevant enough to make the difference between making the nightly batch time frame or not (using full data).

What is non synchronized vs synchronized in Java?

A Synchronized class is a thread-safe class. Non synchronized -It is not-thread safe and can’t be shared between many threads without proper synchronization code. While, Synchronized- It is thread-safe and can be shared with many threads.

What is synchronize vs non synchronized?

There is a difference between static synchronized and non synchronized. If method is non synchronized, it is not safe in multi threading system. When method is synchronized, before the VM starts running that method, it has to acquire a monitor, so only one thread has access to this method at the same time.

How do I make a class thread-safe without synchronization?

It’s also possible to achieve thread-safety using the set of atomic classes that Java provides, including AtomicInteger, AtomicLong, AtomicBoolean and AtomicReference. Atomic classes allow us to perform atomic operations, which are thread-safe, without using synchronization.

How to make static variable thread-safe in Java?

Avoid Global Variables – Unlike local variables, static variables are not automatically thread confined. If you have static variables in your program, then you have to make an argument that only one thread will ever use them, and you have to document that fact clearly. Better, you should eliminate the static variables entirely. Here’s an example: public class PinballSimulator public static PinballSimulator getInstance () return simulator; } } This class has a race in the getInstance() method – two threads could call it at the same time and end up creating two copies of the PinballSimulator object, which we don’t want. To fix this race using the thread confinement approach, you would specify that only a certain thread (maybe the “pinball simulation thread”) is allowed to call PinballSimulator.getInstance(), The risk here is that Java won’t help you guarantee this. In general, static variables are very risky for concurrency. They might be hiding behind an innocuous function that seems to have no side-effects or mutations. Consider this example: /** * @param x integer to test for primeness; requires x > 1 * @return true if x is prime with high probability */ public static boolean isPrime ( int x) private static Map cache = new HashMap (); This function stores the answers from previous calls in case they’re requested again. This technique is called memoization, and it’s a sensible optimization for slow functions like exact primality testing. But now the isPrime method is not safe to call from multiple threads, and its clients may not even realize it. The reason is that the HashMap referenced by the static variable cache is shared by all calls to isPrime(), and HashMap is not threadsafe. If multiple threads mutate the map at the same time, by calling cache.put(), then the map can become corrupted in the same way that the bank account became corrupted in the last reading, If you’re lucky, the corruption may cause an exception deep in the hash map, like a Null­Pointer­Exception or Index­OutOfBounds­Exception, But it also may just quietly give wrong answers, as we saw in the bank account example,

Do threads need to be synchronized?

Introduction – Synchronization in java is the capability to control the access of multiple threads to any shared resource. In the Multithreading concept, multiple threads try to access the shared resources at a time to produce inconsistent results. The synchronization is necessary for reliable communication between threads.

Can we override synchronized method Java?

ID: java/non-sync-override Kind: problem Severity: warning Precision: very-high Tags: – reliability – correctness – concurrency – language-features – external/cwe/cwe-820 Query suites: – java-security-and-quality.qls Click to see the query in the CodeQL repository If a synchronized method is overridden in a subclass, the compiler does not require the overriding method to be synchronized.

Why locks are better than synchronized?

Lock framework vs Thread synchronization in Java

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Thread synchronization mechanism can be achieved using Lock framework, which is present in java.util.concurrent package. Lock framework works like synchronized blocks except locks can be more sophisticated than Java’s synchronized blocks. Locks allow more flexible structuring of synchronized code.

This new approach was introduced in Java 5 to tackle the below-mentioned problem of, Let’s look at an Vector class, which has many synchronized methods. When there are 100 synchronized methods in a class, only one thread can be executed of these 100 methods at any given point in time. Only one thread is allowed to access only one method at any given point of time using a synchronized block.

This is a very expensive operation. Locks avoid this by allowing the configuration of various locks for different purpose. One can have couple of methods synchronized under one lock and other methods under a different lock. This allows more concurrency and also increases overall performance.

• Example: Lock lock = new ReentrantLock(); lock.lock(); // Critical section lock.unlock(); A lock is acquired via the lock() method and released via the unlock() method.
• Invoking an unlock() without lock() will throw an exception.
• As already mentioned the Lock interface is present in java.util.concurrent.locks package and the ReentrantLock implements the Lock interface.

Note: The number of lock() calls should always be equal to the number of unlock() calls. In the below code, the user has created one resource named “TestResource” which has two methods and two different locks for each respectively. There are two jobs named “DisplayJob” and “ReadJob”.

Output : display job display job display job display job display job read job read job read job read job read job Thread 5: TestResource: reading a Job during 4 seconds :: Time – Wed Feb 27 15:49:53 UTC 2019 Thread 0: TestResource: display a Job during 6 seconds :: Time – Wed Feb 27 15:49:53 UTC 2019 Thread 5: The document has been read Thread 6: TestResource: reading a Job during 4 seconds :: Time – Wed Feb 27 15:49:58 UTC 2019 In the above example, DisplayJob not required to wait for ReadJob threads to complete the task as ReadJob and Display job uses two different locks.

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: Lock framework vs Thread synchronization in Java

What is the disadvantage of synchronized method?

Synchronization makes sure that shared resources or data can be accessed by only one thread at a time while execution. its advantage is that it prevent data inconsistency and disadvantage is that it makes execution slower as it makes other thread wait till current thread completes execution.

What is the difference between synchronized and ReentrantLock?

2. Difference between ReentrantLock and synchronized keyword in Java – Though ReentrantLock provides the same visibility and orderings guaranteed as an implicit lock, acquired by synchronized keyword in Java, it provides more functionality and differs in certain aspects.

• As stated earlier, the main difference between synchronized and ReentrantLock is the ability to trying to lock interruptibly, and with a timeout.
• The thread doesn’t need to block infinitely, which was the case with synchronized.
• Let’s see a few more differences between synchronized and Lock-in Java.1) Another significant difference between ReentrantLock and the synchronized keyword is fairness,

The synchronized keyword doesn’t support fairness. Any thread can acquire lock once released, no preference can be specified, on the other hand, you can make ReentrantLock fair by specifying fairness property while creating an instance of ReentrantLock,

• Fairness property provides a lock to the longest waiting thread, in case of contention.2) The second difference between synchronized and Reentrant lock is tryLock() method,
• ReentrantLock provides a convenient tryLock() method, which acquires lock only if its available or not held by any other thread.

This reduces the blocking of thread waiting for lock-in Java applications.3) One more worth noting the difference between ReentrantLock and synchronized keyword in Java is, the ability to interrupt Thread while waiting for Lock, In case of a synchronized keyword, a thread can be blocked waiting for a lock, for an indefinite period of time and there was no way to control that.

1. ReentrantLock provides a method called lockInterruptibly(), which can be used to interrupt thread when it is waiting for lock,
2. Similarly, tryLock() with timeout can be used to timeout if the lock is not available in certain time period.4) ReentrantLock also provides convenient method to get List of all threads waiting for lock.

So, you can see, lot of significant differences between synchronized keyword and ReentrantLock in Java. In short, Lock interface adds lot of power and flexibility and allows some control over lock acquisition process, which can be leveraged to write highly scalable systems in Java.

What is the difference between semaphore and synchronized block in Java?

Semaphore is used to restrict the number of threads that can access a resource. That is, while synchronized allows only one thread to aquire lock and execute the synchonized block / method, Semaphore gives permission up to n threads to go and blocks the others.

How to handle deadlock in Java?

Using Synchronization Objects – Deadlock can be avoided by synchronization and using synchronization primitives. Using synchronization objects, like mutexes or semaphores, is another way to prevent deadlock. This safeguards against deadlocks caused by multiple threads vying for a lock on the same resource.

What is the alternative to synchronized in Java 8?

15. June 2016 To provide synchronized data cache access, I discuss three alternatives in Java 8: synchronized() blocks, ReadWriteLock and StampedLock (new in Java 8). I show code snippets and compare the performance impact on a real world application.

1. Consider the following use case: A data cache that holds key-value pairs and needs to be accessed by several threads concurrently.
2. One option is to use a synchronized container like ConcurrentHashMap or Collections.synchronizedMap(map),
3. Those have their own considerations, but will not be handled in this article.

In our use case, we want to store arbitrary objects into the cache and retrieve them by Integer keys in the range of 0.n. As memory usage and performance is critical in our application, we decided to use a good, old array instead of a more sophisticated container like a Map.

Memory visibility: Threads may see the array in different states (see explanation ).Race conditions: Writing at the same time may cause one thread’s change to be lost (see explanation )

Thus, we need to provide some form of synchronization. To fix the problem of memory visibility, Java’s volatile keyword seems to be the perfect fit. However, making an array volatile has not the desired effect because it makes accesssing the array variable atomic, but not accessing the arrays content,

1. In case the array’s payload is Integer or Long values, you might consider AtomicIntegerArray or AtomicLongArray,
2. But in our case, we want to support arbitrary values, i.e. Objects.
3. Traditionally, there are two ways in Java to do synchronization: synchronized() blocks and ReadWriteLock,
4. Java 8 provides another alternative called StampedLock,

There are propably more exotic ways, but I will focus on these three relatively easy to implement and well understood ways. For each approach, I will provide a short explanation and a code snippet for the cache’s read and write methods. synchronized is a Java keyword that can be used to restrict the execution of code blocks or methods to one thread at a time.

ReadWriteLock is an interface. If I say ReadWriteLock, I mean its only standard library implementation ReentrantReadWriteLock, The basic idea is to have two locks: one for write access and one for read access. While writing locks out everyone else (like synchronized), multiple threads may read concurrently.

StampedLock is a new addition in Java 8. It is similiar to ReadWriteLock in that it also has separate read and write locks. The methods used to aquire locks return a “stamp” (long value), that represents a lock state. I like to think of the stamp as the “version” of the data in terms of data visibility.

This makes a new locking strategy possible: the “optimistic read”. An optimistic read means to aquire a stamp (but no actual lock), read without locking and afterwards validate the lock, i.e. check if it was ok to read without a lock. If we were too optimistic and it turns out someone else wrote in the meantime, the stamp would be invalid.

In this case, we have no choice but to acquire a real read lock and read the value again. Like ReadWriteLock, StampedLock is efficient if there is more read than write access. It can save a lot overhead to not have to acquire and release locks for every read access.

All three alternatives are valid choices for our cache use case, because we expect more reads than writes. To find out which is best, I ran a benchmark with our application. The test machine is a Intel Core i7-5820K CPU which has 6 physical cores (12 logical cores with hyper threading).

Our application spawns 12 threads that access the cache concurrently. The application is a “loader” that imports data from a database, makes calculations and stores the results into a database. The cache is not under stress 100% of the time. However it is vital enough to show a significant impact on the application’s overall runtime.

As benchmark I executed our application with reduced data. To get a good average, I ran each locking strategy three times. Here are the results: In our use case, StampedLock provides the best performance. While 15% difference to synchronized and 24% difference to ReadWriteLock may not seem much, it is relevant enough to make the difference between making the nightly batch time frame or not (using full data).

What is the difference between synchronized method and ReentrantLock in Java?

2. Difference between ReentrantLock and synchronized keyword in Java – Though ReentrantLock provides the same visibility and orderings guaranteed as an implicit lock, acquired by synchronized keyword in Java, it provides more functionality and differs in certain aspects.

• As stated earlier, the main difference between synchronized and ReentrantLock is the ability to trying to lock interruptibly, and with a timeout.
• The thread doesn’t need to block infinitely, which was the case with synchronized.
• Let’s see a few more differences between synchronized and Lock-in Java.1) Another significant difference between ReentrantLock and the synchronized keyword is fairness,

The synchronized keyword doesn’t support fairness. Any thread can acquire lock once released, no preference can be specified, on the other hand, you can make ReentrantLock fair by specifying fairness property while creating an instance of ReentrantLock,

1. Fairness property provides a lock to the longest waiting thread, in case of contention.2) The second difference between synchronized and Reentrant lock is tryLock() method,
2. ReentrantLock provides a convenient tryLock() method, which acquires lock only if its available or not held by any other thread.

This reduces the blocking of thread waiting for lock-in Java applications.3) One more worth noting the difference between ReentrantLock and synchronized keyword in Java is, the ability to interrupt Thread while waiting for Lock, In case of a synchronized keyword, a thread can be blocked waiting for a lock, for an indefinite period of time and there was no way to control that.

1. ReentrantLock provides a method called lockInterruptibly(), which can be used to interrupt thread when it is waiting for lock,
2. Similarly, tryLock() with timeout can be used to timeout if the lock is not available in certain time period.4) ReentrantLock also provides convenient method to get List of all threads waiting for lock.

So, you can see, lot of significant differences between synchronized keyword and ReentrantLock in Java. In short, Lock interface adds lot of power and flexibility and allows some control over lock acquisition process, which can be leveraged to write highly scalable systems in Java.

What are the two types of synchronization in Java?

Summary –

• Synchronization refers to the ability to control the access of multiple threads to any shared resource.
• Java has two types of synchronization methods: 1) Process synchronization and 2) Thread synchronization.
• Lock in Java is built around an internal entity known as a monitor or the lock.
• A Multithreaded program is a method or block protected from interference from other threads sharing the same resource indicated using the `synchronized` keyword.
• Any method that is declared as synchronized is known as a synchronized method.
• In Java, synchronized method locks are accessed on the method, whereas synchronized block locks are accessed on the object.
• In static synchronization, lock access is on the class, not the object and method.
• The main objective of synchronization in Java is to prevent inconsistent data by preventing thread interference.
• The biggest drawback of this method is that all processes are kept in waiting, so the last in the queue must wait until all other processes are complete.

: What is Synchronization in Java? method, block, static type

What is the difference between semaphore and synchronized in Java?

Semaphore is used to restrict the number of threads that can access a resource. That is, while synchronized allows only one thread to aquire lock and execute the synchonized block / method, Semaphore gives permission up to n threads to go and blocks the others.

Jack Gloop