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10_Wiki/Topics/Topic_Programming/Topic_Java/Java_Threads.md

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+---
+id: java-threads
+title: "Java Threads"
+category: "Programming_Language"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["자바 스레드"]
+duplicate_of: ""
+source_trust_level: "B"
+confidence_score: 0.88
+created_at: 2026-07-04
+updated_at: 2026-07-04
+review_reason: ""
+merge_history: []
+tags: ["java", "programming", "w3schools", "threads", "concurrency"]
+raw_sources: ["https://www.w3schools.com/java/java_threads.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[Java Threads]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+Extending `Thread` blocks you from extending any other class (Java has single inheritance), so implementing `Runnable` is the more flexible option specifically because it lets you write `class MyClass extends OtherClass implements Runnable` — the choice between the two thread-creation methods is really a choice about whether you need that inheritance slot for something else. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **Two ways to create a thread** — extend `Thread` and override `run()`, or implement `Runnable` and pass an instance to `new Thread(obj)`. [S1]
+- **`.start()`** — begins running the thread (not calling `run()` directly). [S1]
+- **Extend vs. implement trade-off** — extending Thread blocks further inheritance; implementing Runnable allows extending another class too. [S1]
+- **Concurrency problems** — since threads run in unpredictable order relative to the main program, shared variables can produce unpredictable values. [S1]
+- **`isAlive()`** — checks if a thread is still running; used to wait for a thread to finish before touching shared state. [S1]
+
+## 📖 세부 내용 (Details)
+- Extend Thread: `public class Main extends Thread { public void run() { System.out.println("This code is running in a thread"); } } ... Main thread = new Main(); thread.start();`. [S1]
+- Implement Runnable: `public class Main implements Runnable { public void run() {...} } ... Main obj = new Main(); Thread thread = new Thread(obj); thread.start();`. [S1]
+- Concurrency problem example: `public static int amount = 0; ... thread.start(); System.out.println(amount); amount++; ... // run() { amount++; } — order/value unpredictable`. [S1]
+- Safe wait via isAlive(): `while(thread.isAlive()) { System.out.println("Waiting..."); } System.out.println("Main: " + amount);`. [S1]
+
+## ⚖️ 모순 및 업데이트 (Contradictions & updates)
+- **동시성 문제의 근본 원인**: 스레드와 메인 프로그램이 같은 변수를 동시에 읽고 쓰면 값이 예측 불가능해진다는 점이 명시되고, isAlive()로 대기하는 것이 해결책으로 제시됨. [S1]
+
+## 🛠️ 적용 사례 (Applied in summary)
+현재 발견된 실제 적용 사례가 없습니다 — isAlive()로 스레드 완료를 기다린 후 공유 변수에 접근하는 패턴이 동시성 문제 회피의 실전 사례다. [S1]
+
+## 💻 코드 패턴 (Code patterns)
+Waiting for a thread with isAlive() to avoid concurrency issues (Java):
+```java
+public class Main extends Thread {
+    public static int amount = 0;
+    public static void main(String[] args) {
+        Main thread = new Main();
+        thread.start();
+        while (thread.isAlive()) {
+            System.out.println("Waiting...");
+        }
+        System.out.println("Main: " + amount);
+    }
+    public void run() { amount++; }
+}
+```
+
+## ✅ 검증 상태 및 신뢰도
+- **상태:** draft
+- **검증 단계:** conceptual
+- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
+- **신뢰 점수:** 0.88
+- **중복 검사 결과:** 신규 생성 (New discovery)
+
+## 🔗 지식 그래프 (Knowledge Graph)
+- **상위/루트:** [[Java Tutorial]]
+- **관련 개념:** [[Java Interface]], [[Java Inheritance]], [[Java Lambda]]
+- **참조 맥락:** 병렬 실행과 동시성 문제 — 람다 표현식(Lambda) 챕터로 이어짐.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — Java Threads — https://www.w3schools.com/java/java_threads.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-07-04: Initial draft synthesized from the W3Schools "Java Threads" page (Astra wiki-curation, P-Reinforce v3.1 format).