concurrency-review

Name: concurrency-review
Author: decebals

686stars133forks

How to install concurrency-review?

decebals/claude-code-java/concurrency-review

$npx -y skills add decebals/claude-code-java --skill concurrency-review

Installs into the current project.

Summary

Review Java concurrency code for thread safety, race conditions, deadlocks, and modern patterns (Virtual Threads, CompletableFuture, @Async). Use when user asks "check thread safety", "concurrency review", "async code review", or when reviewing multi-threaded code.

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SKILL.md

1	# Concurrency Review Skill
2
3	Review Java concurrent code for correctness, safety, and modern best practices.
4
5	## Why This Matters
6
7	> Nearly 60% of multithreaded applications encounter issues due to improper management of shared resources. - ACM Study
8
9	Concurrency bugs are:
10	- Hard to reproduce - timing-dependent
11	- Hard to test - may only appear under load
12	- Hard to debug - non-deterministic behavior
13
14	This skill helps catch issues before they reach production.
15
16	## When to Use
17	- Reviewing code with `synchronized`, `volatile`, `Lock`
18	- Checking `@Async`, `CompletableFuture`, `ExecutorService`
19	- Validating thread safety of shared state
20	- Reviewing Virtual Threads / Structured Concurrency code
21	- Any code accessed by multiple threads
22
23	---
24
25	## Modern Java (21/25): Virtual Threads
26
27	### When to Use Virtual Threads
28
29	```java
30	// ✅ Perfect for I/O-bound tasks (HTTP, DB, file I/O)
31	try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {
32	for (Request request : requests) {
33	executor.submit(() -> callExternalApi(request));
34	}
35	}
36
37	// ❌ Not beneficial for CPU-bound tasks
38	// Use platform threads / ForkJoinPool instead
39	```
40
41	Rule of thumb: If your app never has 10,000+ concurrent tasks, virtual threads may not provide significant benefit.
42
43	### Java 25: Synchronized Pinning Fixed
44
45	In Java 21-23, virtual threads became "pinned" when entering `synchronized` blocks with blocking operations. Java 25 fixes this (JEP 491).
46
47	```java
48	// In Java 21-23: ⚠️ Could cause pinning
49	synchronized (lock) {
50	blockingIoCall(); // Virtual thread pinned to carrier
51	}
52
53	// In Java 25: ✅ No longer an issue
54	// But consider ReentrantLock for explicit control anyway
55	```
56
57	### ScopedValue Over ThreadLocal
58
59	```java
60	// ❌ ThreadLocal problematic with virtual threads
61	private static final ThreadLocal<User> currentUser = new ThreadLocal<>();
62
63	// ✅ ScopedValue (Java 21+ preview, improved in 25)
64	private static final ScopedValue<User> CURRENT_USER = ScopedValue.newInstance();
65
66	ScopedValue.where(CURRENT_USER, user).run(() -> {
67	// CURRENT_USER.get() available here and in child virtual threads
68	processRequest();
69	});
70	```
71
72	### Structured Concurrency (Java 25 Preview)
73
74	```java
75	// ✅ Structured concurrency - tasks tied to scope lifecycle
76	try (StructuredTaskScope.ShutdownOnFailure scope = new StructuredTaskScope.ShutdownOnFailure()) {
77	Subtask<User> userTask = scope.fork(() -> fetchUser(id));
78	Subtask<Orders> ordersTask = scope.fork(() -> fetchOrders(id));
79
80	scope.join(); // Wait for all
81	scope.throwIfFailed(); // Propagate exceptions
82
83	return new Profile(userTask.get(), ordersTask.get());
84	}
85	// All subtasks automatically cancelled if scope exits
86	```
87
88	---
89
90	## Spring @Async Pitfalls
91
92	### 1. Forgetting @EnableAsync
93
94	```java
95	// ❌ @Async silently ignored
96	@Service
97	public class EmailService {
98	@Async
99	public void sendEmail(String to) { }
100	}
101
102	// ✅ Enable async processing
103	@Configuration
104	@EnableAsync
105	public class AsyncConfig { }
106	```
107
108	### 2. Calling Async from Same Class
109
110	```java
111	@Service
112	public class OrderService {
113
114	// ❌ Bypasses proxy - runs synchronously!
115	public void processOrder(Order order) {
116	sendConfirmation(order); // Direct call, not async
117	}
118
119	@Async
120	public void sendConfirmation(Order order) { }
121	}
122
123	// ✅ Inject self or use separate service
124	@Service
125	public class OrderService {
126	@Autowired
127	private EmailService emailService; // Separate bean
128
129	public void processOrder(Order order) {
130	emailService.sendConfirmation(order); // Proxy call, async works
131	}
132	}
133	```
134
135	### 3. @Async on Non-Public Methods
136
137	```java
138	// ❌ Non-public methods - proxy can't intercept
139	@Async
140	private void processInBackground() { }
141
142	@Async
143	protected void processInBackground() { }
144
145	// ✅ Must be public
146	@Async
147	public void processInBackground() { }
148	```
149
150	### 4. Default Executor Creates Thread Per Task
151
152	```java
153	// ❌ Default SimpleAsyncTaskExecutor - creates new thread each time!
154	// Can cause OutOfMemoryError under load
155
156	// ✅ Configure proper thread pool
157	@Configuration
158	@EnableAsync
159	public class AsyncConfig {
160
161	@Bean
162	public Executor taskExecutor() {
163	ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
164	executor.setCorePoolSize(10);
165	executor.setMaxPoolSize(50);
166	executor.setQueueCapacity(100);
167	executor.setThreadNamePrefix("async-");
168	executor.setRejectedExecutionHandler(new CallerRunsPolicy());
169	executor.initialize();
170	return executor;
171	}
172	}
173	```
174
175	### 5. SecurityContext Not Propagating
176
177	```java
178	// ❌ SecurityContextHolder is ThreadLocal-bound
179	@Async
180	public void auditAction() {
181	// SecurityContextHol

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concurrency-review

686stars133forks

View on GitHub

How to install concurrency-review?

decebals/claude-code-java/concurrency-review

$npx -y skills add decebals/claude-code-java --skill concurrency-review

Installs into the current project.

Summary

Files · 1

View on GitHub

SKILL.md

1	# Concurrency Review Skill
2
3	Review Java concurrent code for correctness, safety, and modern best practices.
4
5	## Why This Matters
6
7	> Nearly 60% of multithreaded applications encounter issues due to improper management of shared resources. - ACM Study
8
9	Concurrency bugs are:
10	- Hard to reproduce - timing-dependent
11	- Hard to test - may only appear under load
12	- Hard to debug - non-deterministic behavior
13
14	This skill helps catch issues before they reach production.
15
16	## When to Use
17	- Reviewing code with `synchronized`, `volatile`, `Lock`
18	- Checking `@Async`, `CompletableFuture`, `ExecutorService`
19	- Validating thread safety of shared state
20	- Reviewing Virtual Threads / Structured Concurrency code
21	- Any code accessed by multiple threads
22
23	---
24
25	## Modern Java (21/25): Virtual Threads
26
27	### When to Use Virtual Threads
28
29	```java
30	// ✅ Perfect for I/O-bound tasks (HTTP, DB, file I/O)
31	try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {
32	for (Request request : requests) {
33	executor.submit(() -> callExternalApi(request));
34	}
35	}
36
37	// ❌ Not beneficial for CPU-bound tasks
38	// Use platform threads / ForkJoinPool instead
39	```
40
41	Rule of thumb: If your app never has 10,000+ concurrent tasks, virtual threads may not provide significant benefit.
42
43	### Java 25: Synchronized Pinning Fixed
44
45	In Java 21-23, virtual threads became "pinned" when entering `synchronized` blocks with blocking operations. Java 25 fixes this (JEP 491).
46
47	```java
48	// In Java 21-23: ⚠️ Could cause pinning
49	synchronized (lock) {
50	blockingIoCall(); // Virtual thread pinned to carrier
51	}
52
53	// In Java 25: ✅ No longer an issue
54	// But consider ReentrantLock for explicit control anyway
55	```
56
57	### ScopedValue Over ThreadLocal
58
59	```java
60	// ❌ ThreadLocal problematic with virtual threads
61	private static final ThreadLocal<User> currentUser = new ThreadLocal<>();
62
63	// ✅ ScopedValue (Java 21+ preview, improved in 25)
64	private static final ScopedValue<User> CURRENT_USER = ScopedValue.newInstance();
65
66	ScopedValue.where(CURRENT_USER, user).run(() -> {
67	// CURRENT_USER.get() available here and in child virtual threads
68	processRequest();
69	});
70	```
71
72	### Structured Concurrency (Java 25 Preview)
73
74	```java
75	// ✅ Structured concurrency - tasks tied to scope lifecycle
76	try (StructuredTaskScope.ShutdownOnFailure scope = new StructuredTaskScope.ShutdownOnFailure()) {
77	Subtask<User> userTask = scope.fork(() -> fetchUser(id));
78	Subtask<Orders> ordersTask = scope.fork(() -> fetchOrders(id));
79
80	scope.join(); // Wait for all
81	scope.throwIfFailed(); // Propagate exceptions
82
83	return new Profile(userTask.get(), ordersTask.get());
84	}
85	// All subtasks automatically cancelled if scope exits
86	```
87
88	---
89
90	## Spring @Async Pitfalls
91
92	### 1. Forgetting @EnableAsync
93
94	```java
95	// ❌ @Async silently ignored
96	@Service
97	public class EmailService {
98	@Async
99	public void sendEmail(String to) { }
100	}
101
102	// ✅ Enable async processing
103	@Configuration
104	@EnableAsync
105	public class AsyncConfig { }
106	```
107
108	### 2. Calling Async from Same Class
109
110	```java
111	@Service
112	public class OrderService {
113
114	// ❌ Bypasses proxy - runs synchronously!
115	public void processOrder(Order order) {
116	sendConfirmation(order); // Direct call, not async
117	}
118
119	@Async
120	public void sendConfirmation(Order order) { }
121	}
122
123	// ✅ Inject self or use separate service
124	@Service
125	public class OrderService {
126	@Autowired
127	private EmailService emailService; // Separate bean
128
129	public void processOrder(Order order) {
130	emailService.sendConfirmation(order); // Proxy call, async works
131	}
132	}
133	```
134
135	### 3. @Async on Non-Public Methods
136
137	```java
138	// ❌ Non-public methods - proxy can't intercept
139	@Async
140	private void processInBackground() { }
141
142	@Async
143	protected void processInBackground() { }
144
145	// ✅ Must be public
146	@Async
147	public void processInBackground() { }
148	```
149
150	### 4. Default Executor Creates Thread Per Task
151
152	```java
153	// ❌ Default SimpleAsyncTaskExecutor - creates new thread each time!
154	// Can cause OutOfMemoryError under load
155
156	// ✅ Configure proper thread pool
157	@Configuration
158	@EnableAsync
159	public class AsyncConfig {
160
161	@Bean
162	public Executor taskExecutor() {
163	ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
164	executor.setCorePoolSize(10);
165	executor.setMaxPoolSize(50);
166	executor.setQueueCapacity(100);
167	executor.setThreadNamePrefix("async-");
168	executor.setRejectedExecutionHandler(new CallerRunsPolicy());
169	executor.initialize();
170	return executor;
171	}
172	}
173	```
174
175	### 5. SecurityContext Not Propagating
176
177	```java
178	// ❌ SecurityContextHolder is ThreadLocal-bound
179	@Async
180	public void auditAction() {
181	// SecurityContextHol

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