Java synchronization is a crucial aspect of concurrent programming that guarantees the safe interaction of multiple threads with shared resources. Essentially, synchronization serves to avoid race conditions, where the result of operations is influenced by the timing of thread execution. It provides the ability to manage the access of multiple threads to shared resources effectively. Synchronization is particularly useful when restricting access to a shared resource to just one thread is necessary.
Understanding Threads and Shared Resources
A thread in programming symbolizes a distinct route of execution. If multiple threads concurrently interact with common resources, issues can occur because of the unpredictable mixing of operations. For instance, imagine a situation where two threads simultaneously increase a shared variable:
class Counter {
private int count = 0;
public void increment() {
count++;
}
}
When two threads concurrently run the increment function, they could potentially retrieve the current count value, increment it, and update it simultaneously. This simultaneous execution can lead to lost updates or inaccurate final values caused by race conditions.
Introducing Synchronization
In Java, synchronization addresses these issues by allowing only one thread to have exclusive control over either a synchronized code block or method linked to a specific object at any given moment. The main techniques for synchronization in Java involve synchronized methods and synchronized blocks.
Synchronized Methods
In Java, it is possible to designate complete methods as synchronized, which helps in restricting multiple threads from concurrently accessing the method. This approach simplifies the synchronization process as it automatically applies the mechanism to all calls made to the synchronized method.
Example: Synchronized Counter
class SynchronizedCounter {
private int count = 0;
public synchronized void increment() {
count++;
}
public synchronized int getCount() {
return count;
}
}
By implementing this adjustment, simultaneous invocations of increment or getCount will be synchronized, thereby averting race conditions.
Synchronized Blocks
A synchronized block ensures that only one thread can access shared resources at a time, providing exclusive access. The structure of a synchronized block is as follows:
synchronized (object) {
// Synchronized code block
}
The monitor object, also known as a lock, is the focus here. It allows only one thread to hold a lock on it at a time. If other threads wish to access synchronized blocks associated with this object, they must wait until the lock is released.
Intrinsic Locks and Synchronization
In Java, each object comes with an inherent lock known as an intrinsic lock or monitor lock. When a thread accesses a synchronized block or method, it acquires the lock specific to that object. Subsequently, no other thread can access the synchronized block or method for the object until the lock is relinquished.
Deadlocks
While synchronization helps prevent race conditions, it can also introduce deadlocks if not carefully managed. Deadlocks can occur when multiple threads are indefinitely waiting for resources from one another, resulting in a standstill. To prevent deadlocks, organize locks and release them in the opposite order in which they were acquired.
Locking Granularity
Choosing the optimal locking granularity is crucial to prevent contention and enhance performance. Overgeneralizing can diminish concurrency, while being overly specific can result in increased overhead. It is essential to pinpoint the specific part of the code that requires exclusive access to shared resources and synchronize only that particular section.
Concurrent Collections
Within Java, there exist versions of the standard collections classes that are thread-safe and reside in the java.util.concurrent package. Examples of such classes include ConcurrentHashMap and ConcurrentLinkedQueue. These particular classes incorporate built-in synchronization mechanisms to ensure thread safety, all while maintaining control over synchronization within the classes themselves.
Volatile Keyword
Moreover, the volatile keyword can be employed to ensure that modifications to variables are visible across threads. When variables are marked as volatile, their value is always retrieved directly from memory, and any updates are immediately visible to all other threads. Nevertheless, it's essential to note that volatile doesn't provide atomicity for compound operations like increments.
Atomic Classes
Java provides atomic classes in the java.util.concurrent.atomic package such as Atomic Integer and Atomic Long. These classes enable atomic operations on variables without the need for explicit synchronization. They leverage the low-level atomic operations of hardware to ensure thread safety.
Why use Synchronization?
Synchronization is primarily utilized to prevent thread conflicts and ensure consistency in programming.
Types of Synchronization
There exist two primary forms of synchronization in computing:
- Process Synchronization
- Thread Synchronization
In this section, we will exclusively focus on the synchronization of threads.
Thread Synchronization
There are two types of thread synchronization in Java: mutual exclusive and inter-thread communication.
- Mutual Exclusive Synchronized method. Synchronized block. Static synchronization.
- Cooperation (Inter-thread communication in Java)
- Synchronized method.
- Synchronized block.
- Static synchronization.
Mutual Exclusive
Mutual Exclusive helps keep threads from interfering with one another while sharing data. It can be achieved by using the following three ways:
- By Using Synchronized Method
- By Using Synchronized Block
- By Using Static Synchronization
Concept of Lock in Java
In the context of synchronization, a fundamental element called a lock or monitor is utilized. Each object possesses a corresponding lock. It is customary for a thread requiring coherent access to an object's fields to first obtain the object's lock before interacting with them, and subsequently relinquish the lock upon completion.
Starting from Java 5, the java.util.concurrent.locks package includes various implementations of locks.
Understanding the Problem without Synchronization
In this particular instance, synchronization is lacking, resulting in inconsistent output. Let's examine the case:
Example
class Table {
// Method to print the table, not synchronized
void printTable(int n) {
for(int i = 1; i <= 5; i++) {
// Print the multiplication result
System.out.println(n * i);
try {
// Pause execution for 400 milliseconds
Thread.sleep(400);
} catch(Exception e) {
// Handle any exceptions
System.out.println(e);
}
}
}
}
class MyThread1 extends Thread {
Table t;
// Constructor to initialize Table object
MyThread1(Table t) {
this.t = t;
}
// Run method to execute thread
public void run() {
// Call printTable method with argument 5
t.printTable(5);
}
}
class MyThread2 extends Thread {
Table t;
// Constructor to initialize Table object
MyThread2(Table t) {
this.t = t;
}
// Run method to execute thread
public void run() {
// Call printTable method with argument 100
t.printTable(100);
}
}
public class Main {
public static void main(String args[]) {
// Create a Table object
Table obj = new Table();
// Create MyThread1 and MyThread2 objects with the same Table object
MyThread1 t1 = new MyThread1(obj);
MyThread2 t2 = new MyThread2(obj);
// Start both threads
t1.start();
t2.start();
}
}
Output:
5
100
10
200
15
300
20
400
25
500
Java Synchronized Method
When you designate a method as synchronized, it is referred to as a synchronized method.
A synchronized method is employed to ensure exclusive access to an object for any shared resource.
When a thread calls a synchronized method, it automatically obtains the lock for the object and releases it once the thread finishes its execution.
Example
class Table {
// Synchronized method to print the table
synchronized void printTable(int n) {
for(int i = 1; i <= 5; i++) {
// Print the multiplication result
System.out.println(n * i);
try {
// Pause execution for 400 milliseconds
Thread.sleep(400);
} catch(Exception e) {
// Handle any exceptions
System.out.println(e);
}
}
}
}
class MyThread1 extends Thread {
Table t;
// Constructor to initialize Table object
MyThread1(Table t) {
this.t = t;
}
// Run method to execute thread
public void run() {
// Call synchronized method printTable with argument 5
t.printTable(5);
}
}
class MyThread2 extends Thread {
Table t;
// Constructor to initialize Table object
MyThread2(Table t) {
this.t = t;
}
// Run method to execute thread
public void run() {
// Call synchronized method printTable with argument 100
t.printTable(100);
}
}
public class Main {
public static void main(String args[]) {
// Create a Table object
Table obj = new Table();
// Create MyThread1 and MyThread2 objects with the same Table object
MyThread1 t1 = new MyThread1(obj);
MyThread2 t2 = new MyThread2(obj);
// Start both threads
t1.start();
t2.start();
}
}
Output:
5
10
15
20
25
100
200
300
400
500
Example of Synchronized Method by Using Anonymous Class
Within this software, a pair of threads has been established through the utilization of an anonymous class, thereby reducing the amount of code needed.
Example
// Program of synchronized method by using anonymous class
class Table {
// Synchronized method to print the table
synchronized void printTable(int n) {
for(int i = 1; i <= 5; i++) {
// Print the multiplication result
System.out.println(n * i);
try {
// Pause execution for 400 milliseconds
Thread.sleep(400);
} catch(Exception e) {
// Handle any exceptions
System.out.println(e);
}
}
}
}
public class Main {
public static void main(String args[]) {
// Create a Table object
final Table obj = new Table(); // Only one object
// Create thread t1 using anonymous class
Thread t1 = new Thread() {
public void run() {
// Call synchronized method printTable with argument 5
obj.printTable(5);
}
};
// Create thread t2 using anonymous class
Thread t2 = new Thread() {
public void run() {
// Call synchronized method printTable with argument 100
obj.printTable(100);
}
};
// Start both threads
t1.start();
t2.start();
}
}
Output:
5
10
15
20
25
100
200
300
400
500
Advantages of Synchronization in Java
Ensuring Thread Safety: Synchronization guarantees that concurrent access to shared resources is controlled, allowing only one thread to access them at a time. By doing so, it prevents race conditions and upholds the integrity of data, thus simplifying the development of multi-threaded applications by eliminating concerns about unpredictable outcomes resulting from simultaneous access.
Maintaining Consistency: Synchronization plays a vital role in guaranteeing that simultaneous actions on shared resources are carried out consistently and predictably. This is essential for upholding the accuracy of the program's logic and averting unforeseen results.
Ensuring Data Visibility: Synchronization techniques like locks and memory barriers are put in place to ensure that modifications made by one thread to shared variables are observable by other threads. This guarantees that threads consistently access the latest values of shared data, thus avoiding discrepancies caused by outdated data.
Deadlock Prevention: In the realm of synchronization, there are strategies available to avert deadlocks, which occur when multiple threads are stuck indefinitely, each waiting for the other to free up resources. Adhering to recommended methodologies like maintaining a uniform lock acquisition sequence and implementing timeouts can help reduce the likelihood of deadlocks occurring within your multi-threaded software.
Synchronization plays a crucial role in coordinating and facilitating communication among threads by enabling them to pause until specific conditions are fulfilled before moving forward. This functionality supports the utilization of synchronization tools like semaphores, mutexes, and barriers, which are vital components in the development of intricate multi-threaded algorithms.
Optimizing Resource Usage: Even though synchronization can introduce extra work to multi-threaded applications through locking and context switching, it plays a crucial role in maximizing the efficient use of shared resources like CPU time, memory, and I/O devices. Through enabling multiple threads to collaborate without causing conflicts, synchronization enhances the overall performance and responsiveness of the software.
Legacy Code Compatibility: Synchronization plays a crucial role in Java concurrency and is extensively utilized in libraries, frameworks, and established codebases. Through synchronization, developers can maintain compatibility with older code and libraries that depend on secure multi-threading practices.
Disdvantages of Synchronization in Java
Performance Impact: When synchronization is implemented, it requires acquiring and releasing locks. This action introduces overhead because it involves context switching and competition for shared resources. In scenarios with high concurrency, where multiple threads compete for identical locks, this overhead can significantly reduce performance.
The possibility of Deadlocks: Improper utilization of synchronization mechanisms could result in deadlocks, a situation where threads are stuck indefinitely, anticipating the release of locks by each other. Deadlocks can be difficult to troubleshoot and may lead to the suspension of the entire application, affecting its availability and dependability.
Decreased Scalability: The scalability of multi-threaded applications can be hindered by synchronization, which can create bottlenecks. When multiple threads vie for the same locks, they might experience significant waiting periods, diminishing overall throughput and scalability, especially on systems with numerous CPU cores.
The complexity and maintenance of synchronized code can surpass that of single-threaded or lock-free options. Tasks such as handling locks, guaranteeing correct lock acquisition and release, and preventing deadlocks demand meticulous planning and testing, leading to heightened intricacy and maintenance overhead in the codebase.
Possibility of Livelocks: Livelocks, although similar to deadlocks, arise when threads consistently alter their statuses in reaction to one another, hindering any advancement. Livelocks may manifest when threads repetitively obtain and release locks following a particular sequence without moving forward in resolving the conflict.
Debugging synchronization-related problems like race conditions, deadlocks, and livelocks can pose significant challenges, particularly in intricate multi-threaded programs. These issues may arise intermittently and could prove difficult to replicate in controlled settings, thereby complicating the process of identifying and resolving them.
Reduced Concurrency: Excessive utilization of synchronization mechanisms can result in a reduction of concurrency since threads might end up waiting more for locks rather than executing productive tasks. Employing fine-grained locking can help address this problem; however, it can elevate intricacy and potentially add extra processing burden.
Performance Impact of I/O Operations: The synchronization process can potentially result in decreased performance if threads are blocked during I/O operations while maintaining locks. This situation can lead to additional threads being blocked unnecessarily, consequently lowering the overall throughput and responsiveness of the system.
Synchronization in Java MCQ
- What is the primary benefit of using a synchronized method over a synchronized block?
- Synchronized methods are faster than synchronized blocks.
- Synchronized methods reduce the risk of thread interference and memory consistency errors.
- Synchronized methods can synchronize on different objects.
- Synchronized blocks cannot be used within methods.
Explanation: Synchronized methods provide a simpler way to ensure that a method can be executed by only one thread at a time, thereby reducing the risk of thread interference and memory consistency errors.
- How does the java.util.concurrent.locks.ReentrantLock differ from the synchronized keyword?
- ReentrantLock does not provide the same level of thread safety.
- ReentrantLock does not support reentrancy.
- ReentrantLock provides more flexibility and features like timed lock waits and interruptible lock waits.
- ReentrantLock is slower and less efficient than the synchronized keyword.
Explanation: ReentrantLock offers additional capabilities over the synchronized keyword, such as the ability to interrupt a thread waiting for a lock or the ability to try to acquire the lock without blocking indefinitely.
- What would happen if a thread holding a lock does not release it and exits the synchronized block or method due to an exception?
- The lock is not released, causing a potential deadlock.
- The lock is automatically released.
- Another thread can still acquire the lock.
- The JVM crashes.
Explanation: In Java, the synchronized keyword ensures that the lock is automatically released when the thread exits the synchronized block or method, even if it exits due to an exception.
- What is the potential downside of using too much synchronization in a Java program?
- Increased complexity in code management.
- Potential deadlocks.
- Reduced performance due to thread contention.
- All of the above.
Explanation: Overusing synchronization can lead to increased code complexity, potential deadlocks, and reduced performance due to threads contending for the same locks, thus negatively impacting the program's efficiency.
- Which of the following statements is true regarding static synchronization in Java?
- Static synchronization locks the object instance on which the static method is called.
- Static synchronization locks the class object associated with the class.
- Static synchronization is not supported in Java.
- Static synchronization is identical to instance method synchronization.
When a method is declared as static synchronized, it locks the class object rather than a specific instance of the class. This guarantees that only one thread can access the synchronized method at a time, regardless of the number of class instances.