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-**Read and write operations** on primitive types like `int`, `short`, `byte`, `char`, `float`, and `boolean` are
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atomic, except for `long` and `double` on some architectures. On 64-bit architectures, `long` and `double` are
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typically atomic, but on 32-bit architectures, they might not be.
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Atomic operations are fundamental building blocks in concurrent programming that ensure thread-safe updates to shared variables. Let's explore how they work and why they're essential for reliable multi-threaded applications.
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2.**Volatile Variables**:
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- Declaring a variable `volatile` ensures that reads and writes to that variable are atomic. Additionally, it ensures
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visibility, meaning changes to a volatile variable by one thread are immediately visible to other threads.
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## What Makes an Operation Atomic?
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3.**Atomic Classes in `java.util.concurrent.atomic`**:
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- Java provides classes in the `java.util.concurrent.atomic` package for performing atomic operations on variables.
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These classes include:
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-`AtomicInteger`
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-`AtomicLong`
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-`AtomicBoolean`
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-`AtomicReference`
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- These classes provide methods like `get()`, `set()`, `incrementAndGet()`, `compareAndSet()`, etc., which are
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implemented using low-level atomic instructions provided by the hardware, ensuring atomicity.
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An atomic operation is one that executes as a single, uninterruptible unit. Just like a transaction in a database, either the entire operation completes successfully, or nothing changes at all.
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### Why Atomicity is Important
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##Primitive Types
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Atomicity is essential in multi-threaded environments to ensure that operations are completed without interference.
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Without atomic operations, multiple threads could read and write shared data simultaneously, leading to inconsistent and
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unpredictable results.
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✅ Basic types (`int`, `short`, `byte`, `char`, `float`, and `boolean`) have atomic read/write operations
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❌ `long` and `double` require special handling:
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⚠️ Not atomic on 32-bit systems
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✅ Usually atomic on 64-bit systems
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💡 Use `volatile` keyword to ensure atomicity
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### Example: Using `AtomicInteger`
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##Volatile Variables
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Here's an example of using `AtomicInteger` to perform atomic operations:
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🔄 Ensures immediate visibility across threads
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📝 Guarantees atomic read/write operations
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🔄 Prevents compiler optimizations that could break thread safety
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## Atomic Classes
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```java
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AtomicInteger// For integers
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AtomicLong// For long values
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AtomicBoolean// For boolean flags
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AtomicReference// For object references
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```
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Common atomic methods:
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🔄 `get()` - Atomic read operation
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➕ `incrementAndGet()` - Atomically increment and retrieve
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↔️ `compareAndSet()` - Compare value and atomically set new value if match
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## Hardware Support
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Modern processors provide specialized instructions for atomic operations:
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### Compare-and-Swap (CAS)
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💪 Compares a memory location's value with expected value
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💪 Only updates if values match
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💪 Executes as a single, uninterruptible operation
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### Load-Link/Store-Conditional
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🔗 Load-link reads value and marks memory location
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🔗 Store-conditional writes only if no other thread modified the location
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🔗 Ensures atomicity through hardware-level locking
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## Practical Example
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```kotlin
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importjava.util.concurrent.atomic.AtomicInteger
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@JvmStatic
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funmain(args:Array<String>) {
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// Create two threads performing 1000 increments each
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val task =Runnable {
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for (i in0 until 1000) {
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counter.incrementAndGet()
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val t1 =Thread(task)
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val t2 =Thread(task)
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// Start threads
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t1.start()
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t2.start()
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try {
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t1.join()
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t2.join()
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} catch (e:InterruptedException) {
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e.printStackTrace()
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}
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// Wait for completion
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t1.join()
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t2.join()
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println("Counter: ${counter.get()}")
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// Should print 2000 (1000 increments × 2 threads)
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println("Final counter value: ${counter.get()}")
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}
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}
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```
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Atomic operations in the JVM are implemented using low-level mechanisms provided by the hardware and the operating
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system. These mechanisms ensure that atomic operations are performed without interference from other threads. Here is a
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detailed look at how atomic operations work under the hood:
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## Best Practices
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### Use atomic operations when:
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🔒 Updating shared variables in multi-threaded environments
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🔒 Implementing thread-safe counters or flags
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🔒 Building lock-free data structures
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Hardware Support
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Modern processors provide specific instructions that ensure atomicity. These instructions are part of the CPU's
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instruction set and are used to perform atomic read-modify-write operations. Examples of such instructions include:
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### Avoid atomic operations when:
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* Compare-and-Swap (CAS): This instruction compares the value at a memory location with an expected value and, only if
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they match, modifies the memory location to a new value. This is done atomically, ensuring no other thread can interfere
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during the operation.
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⚠️ Working with thread-local variables
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⚠️ Performance is critical and synchronization isn't needed
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⚠️ Using synchronized blocks would be more readable
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* Load-Link/Store-Conditional (LL/SC): These instructions work together to provide atomicity. The load-link reads the
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value, and the store-conditional writes the value only if no other write has occurred to the memory location since the
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load-link.
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Remember that atomic operations provide a powerful tool for building thread-safe applications, but they shouldn't replace proper synchronization entirely. Choose the right concurrency primitive based on your specific requirements and performance constraints.
In concurrency, the **happens-before** relationship is a fundamental concept used to ensure that operations are
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performed in a predictable and safe order. It helps define the ordering constraints between different operations in a
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concurrent program, ensuring that certain operations are completed before others begin. Here’s a breakdown of the
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concept:
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## 📌 Overview
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In concurrency, the **happens-before** relationship is a fundamental concept used to ensure that operations are **performed in a predictable and safe order**. It defines ordering constraints between different operations in a concurrent program, ensuring that certain operations are **completed before others begin**.
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## 1. Definition
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---
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The happens-before relationship is a partial ordering of operations in a concurrent system that guarantees that if one
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operation A happens-before another operation B, then the effects of A are visible to B. In other words, if operation A
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happens-before operation B, then B will see the results of A.
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## 1️⃣ Definition
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The **happens-before** relationship is a **partial ordering** of operations in a concurrent system. It guarantees that:
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## 2. Why It Matters
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✔ If **operation A happens-before operation B**, then the **effects of A are visible to B**.
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✔ If there is **no happens-before relationship**, then **no guarantee** exists regarding the order or visibility of changes.
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The happens-before relationship is crucial for understanding and reasoning about concurrency issues such as race
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conditions, visibility of changes across threads, and ordering of operations. It helps ensure consistency and
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correctness in concurrent programs.
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📝 **Example:** If one thread writes a variable, and another thread reads it, the happens-before relationship ensures the read operation sees the updated value.
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## 3. Rules and Examples
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---
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Here are some common rules and examples of happens-before relationships:
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## 2️⃣ Why It Matters
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The happens-before relationship is crucial for understanding and preventing concurrency issues like:
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-**Race conditions** ⚠ (Ensuring that no two threads interfere with each other unpredictably)
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-**Visibility of changes** 🔍 (Ensuring that changes made by one thread are visible to another)
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-**Ordering of operations** 🔄 (Ensuring that operations execute in the intended sequence)
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Without a proper happens-before relationship, one thread **might not see updates** from another, leading to unpredictable behavior.
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---
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## 3️⃣ Rules and Examples
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### 🔹 **Rule 1: Program Order Rule**
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Within a **single thread**, statements execute **in the order they appear** in the code.
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📝 **Example:**
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```java
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int x =5; // Happens-before the next statement
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int y = x +2; // y will correctly see x as 5
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### 🔹 Rule2:MonitorLockRule
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Unlocking a synchronized block happens-before any subsequent locking of the same monitor.
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📝 **Example:**
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```java
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synchronized (lock) {
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sharedValue =42; // Happens-before another thread locks and reads sharedValue
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}
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```
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### 🔹 Rule 3: Volatile Variable Rule
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A write to a volatile variable happens-before any subsequent read of the same variable.
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📝 **Example:**
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```java
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volatileboolean flag =false;
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Thread1:
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flag =true; // Happens-before Thread 2 reads it
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Thread2:
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if (flag) {
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System.out.println("Flag is true!"); // Guaranteed to see updated value
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}
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```
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### 🔹 Rule 4: Thread Start Rule
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A call to Thread.start() on a new thread happens-before any actions within that thread.
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📝 **Example:**
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```java
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Thread t =newThread(() ->System.out.println("Thread started!"));
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t.start(); // Happens-before the message is printed
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```
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### 🔹 Rule 5: Thread Termination Rule
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A thread’s termination happens-before another thread detecting it via Thread.join().
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📝 **Example:**
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```java
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Thread t =newThread(() ->System.out.println("Task complete"));
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t.start();
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t.join(); // Happens-before main thread continues execution
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System.out.println("Thread has finished");
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```
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### 🔹 Rule 6: Inter-Thread Communication (Executor & Future)
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A result written using Future.set() happens-before another thread retrieving the value via Future.get().
Problem: Starvation occurs when a thread is perpetually denied necessary resources to proceed with its execution because
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other threads keep consuming those resources.
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# 🚀 Starvation in Java: Understanding the Problem and Solutions
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## 🔹 What is Starvation?
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**Starvation** occurs when a thread is **perpetually denied access** to necessary resources, preventing it from making progress. This typically happens when **higher-priority threads or resource-hogging threads** continuously consume the available CPU or synchronization locks, leaving lower-priority threads **waiting indefinitely**.
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---
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## 🔥 Problem: Why Does Starvation Occur?
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Starvation happens when:
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1️⃣ **Thread Priority Imbalance**
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- Threads with **higher priority** keep executing, while lower-priority threads get **little to no CPU time**.
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- Example: A **high-priority** thread is always scheduled before a **low-priority** thread.
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2️⃣ **Unfair Locking (Biased Locking)**
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- A thread might **never acquire a lock** if other threads keep **acquiring and releasing** it quickly.
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- Example: A thread waiting for a **synchronized** block but never gets a turn because other threads **continuously hold the lock**.
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3️⃣ **Unfair Resource Allocation**
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- Shared resources are **continuously consumed** by a few threads, **preventing** other threads from using them.
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- Example: A **thread pool** where some worker threads **always get tasks**, while others remain idle.
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4️⃣ **Frequent Lock Contention**
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- If a lock is frequently **acquired and released** by the same few threads, **other threads may never get access**.
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- Example: A **database connection pool** where a few connections are always occupied by the same threads.
Synchronized is a keyword in Java that is used to mark a method or a block of code as synchronized. When a method is marked as synchronized, the thread that is executing it is granted a lock on the object the method belongs to. This means that no other thread can execute any synchronized method on the same object until the current thread releases the lock. This is a simple and effective way to ensure that only one thread can access a resource at a time.
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How Synchronization Works
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Method-Level Synchronization: When a method is declared as synchronized, the thread holds the lock for the object on which the method is called. Other threads cannot call any synchronized method on the same object until the lock is released.
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Block-Level Synchronization: A block of code can be synchronized using the synchronized keyword. The thread holds the lock for the specified object within the block. This allows more fine-grained control over synchronization.
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Intrinsic Locks: Every object in Java has an intrinsic lock (or monitor lock). When a thread enters a synchronized method or block, it acquires the intrinsic lock for that object. When it exits the method or block, it releases the lock.
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Reentrant Locks: Java's intrinsic locks are reentrant. This means that if a thread already holds the lock for an object, it can re-enter any synchronized method or block on that object without deadlocking.
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Thread Safety: Synchronized methods and blocks ensure that only one thread can execute them at a time on a given object, preventing concurrent access issues and ensuring thread safety.
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Benefits of Synchronized
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Thread Safety: Synchronized methods ensure that only one thread can execute them at a time on a given object, preventing data corruption and ensuring consistency.
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Atomicity: Synchronized blocks provide atomicity, ensuring that a series of operations are executed as a single, indivisible step.
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Visibility: Changes made by one thread to a synchronized block are visible to other threads, ensuring that they see the most up-to-date state of shared variables.
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# Synchronized in JVM
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## What is Synchronized?
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`Synchronized` is a keyword in Java that is used to mark a method or a block of code as **synchronized**. When a method is marked as synchronized, the thread that is executing it is granted a **lock** on the object the method belongs to.
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This means that **no other thread** can execute any synchronized method on the same object until the current thread releases the lock. It ensures that only one thread can access a **shared resource** at a time.
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---
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## 🔹 How Synchronization Works
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### 1️⃣ Method-Level Synchronization
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- When a **method** is declared as `synchronized`, the **thread** holds the lock for the **object** on which the method is called.
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- Other threads **cannot call** any synchronized method on the same object **until the lock is released**.
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### 2️⃣ Block-Level Synchronization
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- A **block of code** can be synchronized using the `synchronized` keyword.
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- The **thread holds the lock** for the specified object **only within** the block.
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- This allows **fine-grained** control over synchronization.
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### 3️⃣ Intrinsic Locks
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- Every object in Java has an **intrinsic lock** (or **monitor lock**).
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- When a thread enters a **synchronized method or block**, it **acquires** the **intrinsic lock** for that object.
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- When it exits the method or block, it **releases** the lock.
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### 4️⃣ Reentrant Locks
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- Java's **intrinsic locks** are **reentrant**.
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- If a thread **already holds the lock** for an object, it can **re-enter** any synchronized method or block on that object **without deadlocking**.
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### 5️⃣ Thread Safety
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- Synchronized methods and blocks **ensure** that **only one thread** can execute them at a time on a given object.
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- This prevents **concurrent access issues** and ensures **thread safety**.
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---
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## 🔹 Benefits of Synchronized
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✔ **Thread Safety**:
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Synchronized methods ensure that **only one thread** can execute them at a time on a given object, **preventing data corruption** and **ensuring consistency**.
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✔ **Atomicity**:
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Synchronized blocks **provide atomicity**, ensuring that a **series of operations** are executed as a **single, indivisible step**.
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✔ **Visibility**:
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Changes made by one thread to a synchronized block **are visible to other threads**, ensuring they see the **most up-to-date state** of shared variables.
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