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# Concurrent Utilities
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Apache Commons Lang provides robust concurrency utilities for thread-safe initialization, background processing, circuit breakers, and fault tolerance. These utilities simplify concurrent programming patterns while providing proper error handling and resource management.
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## Core Concurrent Classes
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### ConcurrentUtils - General Concurrency Utilities
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Provides utility methods for safe concurrent operations and exception handling:
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```java { .api }
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import org.apache.commons.lang3.concurrent.ConcurrentUtils;
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```
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#### Safe Initialization Patterns
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```java { .api }
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// Safe initialization with exception handling
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public static <T> T initialize(ConcurrentInitializer<T> initializer) throws ConcurrentException
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public static <T> T initializeUnchecked(ConcurrentInitializer<T> initializer)
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// Exception handling utilities
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public static ConcurrentException extractCause(ExecutionException ex)
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public static ConcurrentRuntimeException extractCauseUnchecked(ExecutionException ex)
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public static void handleCause(ExecutionException ex) throws ConcurrentException
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public static void handleCauseUnchecked(ExecutionException ex)
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// Map operations
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public static <K, V> V createIfAbsent(ConcurrentMap<K, V> map, K key, ConcurrentInitializer<V> init) throws ConcurrentException
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public static <K, V> V createIfAbsentUnchecked(ConcurrentMap<K, V> map, K key, ConcurrentInitializer<V> init)
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public static <K, V> V putIfAbsent(ConcurrentMap<K, V> map, K key, V value)
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```
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**Usage Examples:**
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```java { .api }
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public class ConcurrentUtilsExamples {
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    private final ConcurrentMap<String, DatabaseConnection> connections = new ConcurrentHashMap<>();
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    // Safe map operations
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    public DatabaseConnection getConnection(String name) throws ConcurrentException {
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        return ConcurrentUtils.createIfAbsent(connections, name, () -> {
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            // This initialization happens only once per key
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            return DatabaseConnectionFactory.create(name);
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        });
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    }
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    // Unchecked version (wraps exceptions in runtime exceptions)
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    public DatabaseConnection getConnectionUnchecked(String name) {
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        return ConcurrentUtils.createIfAbsentUnchecked(connections, name, () -> {
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            return DatabaseConnectionFactory.create(name);
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        });
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    }
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    // Safe Future handling
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    public <T> T getResultSafely(Future<T> future) throws ConcurrentException {
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        try {
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            return future.get();
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        } catch (ExecutionException e) {
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            ConcurrentUtils.handleCause(e);
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            return null; // Never reached
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        } catch (InterruptedException e) {
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            Thread.currentThread().interrupt();
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            throw new ConcurrentException("Thread interrupted", e);
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        }
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    }
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}
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```
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### Initializer Classes - Thread-Safe Lazy Initialization
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#### AtomicInitializer - Single-Use Atomic Initialization
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```java { .api }
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import org.apache.commons.lang3.concurrent.AtomicInitializer;
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```
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```java { .api }
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public class AtomicInitializerExample {
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    // Expensive object that should be created only once
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    private final AtomicInitializer<DatabaseConnectionPool> poolInitializer = 
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        new AtomicInitializer<DatabaseConnectionPool>() {
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            @Override
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            protected DatabaseConnectionPool initialize() throws ConcurrentException {
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                // This method is called exactly once
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                return new DatabaseConnectionPool(
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                    "jdbc:postgresql://localhost/mydb",
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                    "user", "password", 10
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                );
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            }
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        };
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    public DatabaseConnectionPool getConnectionPool() throws ConcurrentException {
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        return poolInitializer.get(); // Thread-safe, initializes only once
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    }
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    // Alternative using builder pattern (Java 8+)
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    private final AtomicInitializer<ConfigurationManager> configInitializer =
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        AtomicInitializer.<ConfigurationManager>builder()
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            .setInitializer(() -> ConfigurationManager.loadFromFile("config.properties"))
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            .get();
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}
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```
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#### LazyInitializer - Thread-Safe Lazy Initialization
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```java { .api }
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import org.apache.commons.lang3.concurrent.LazyInitializer;
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```
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```java { .api }
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public class LazyInitializerExample {
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    // Cache that's initialized on first access
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    private final LazyInitializer<Cache<String, Object>> cacheInitializer = 
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        new LazyInitializer<Cache<String, Object>>() {
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            @Override
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            protected Cache<String, Object> initialize() throws ConcurrentException {
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                return CacheBuilder.newBuilder()
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                    .maximumSize(1000)
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                    .expireAfterWrite(30, TimeUnit.MINUTES)
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                    .build();
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            }
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        };
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    public Cache<String, Object> getCache() throws ConcurrentException {
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        return cacheInitializer.get();
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    }
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    // Service registry example
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    private final LazyInitializer<ServiceRegistry> registryInitializer = 
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        LazyInitializer.<ServiceRegistry>builder()
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            .setInitializer(() -> {
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                ServiceRegistry registry = new ServiceRegistry();
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                registry.registerService("userService", new UserServiceImpl());
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                registry.registerService("orderService", new OrderServiceImpl());
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                return registry;
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            })
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            .get();
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}
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```
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#### ConstantInitializer - Simple Constant Value Holder
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```java { .api }
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import org.apache.commons.lang3.concurrent.ConstantInitializer;
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```
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```java { .api }
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public class ConstantInitializerExample {
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    // For pre-computed values
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    private final ConcurrentInitializer<String> appNameInitializer = 
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        new ConstantInitializer<>("MyApplication v1.0");
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    // For configuration values
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    private final ConcurrentInitializer<Integer> maxUsersInitializer =
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        new ConstantInitializer<>(Integer.parseInt(System.getProperty("max.users", "1000")));
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    public String getApplicationName() throws ConcurrentException {
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        return appNameInitializer.get();
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    }
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    public int getMaxUsers() throws ConcurrentException {
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        return maxUsersInitializer.get();
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    }
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}
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```
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### Background Processing
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#### BackgroundInitializer - Asynchronous Initialization
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```java { .api }
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import org.apache.commons.lang3.concurrent.BackgroundInitializer;
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```
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```java { .api }
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public class BackgroundProcessingExample {
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    // Initialize expensive resources in background
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    private final BackgroundInitializer<SearchIndex> searchIndexInitializer = 
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        new BackgroundInitializer<SearchIndex>() {
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            @Override
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            protected SearchIndex initialize() throws Exception {
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                // This runs in a background thread
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                SearchIndex index = new SearchIndex();
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                index.loadFromDatabase(); // Expensive operation
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                return index;
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            }
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        };
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    public void startBackgroundInitialization() {
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        // Start background initialization immediately
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        searchIndexInitializer.start();
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    }
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    public SearchIndex getSearchIndex() throws ConcurrentException {
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        // This will block until background initialization is complete
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        return searchIndexInitializer.get();
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    }
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    // Custom executor example
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    private final ExecutorService customExecutor = Executors.newFixedThreadPool(2);
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    private final BackgroundInitializer<ReportGenerator> reportInitializer =
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        BackgroundInitializer.<ReportGenerator>builder()
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            .setExecutor(customExecutor)
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            .setInitializer(() -> {
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                ReportGenerator generator = new ReportGenerator();
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                generator.preloadTemplates();
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                return generator;
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            })
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            .get();
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}
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```
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#### MultiBackgroundInitializer - Multiple Background Tasks
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```java { .api }
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import org.apache.commons.lang3.concurrent.MultiBackgroundInitializer;
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```
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```java { .api }
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public class MultiBackgroundExample {
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    public void initializeApplication() throws ConcurrentException {
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        MultiBackgroundInitializer initializer = new MultiBackgroundInitializer();
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        // Add multiple background tasks
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        initializer.addInitializer("database", new BackgroundInitializer<DataSource>() {
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            @Override
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            protected DataSource initialize() throws Exception {
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                return createDataSource();
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            }
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        });
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        initializer.addInitializer("cache", new BackgroundInitializer<CacheManager>() {
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            @Override
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            protected CacheManager initialize() throws Exception {
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                return createCacheManager();
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            }
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        });
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        initializer.addInitializer("search", new BackgroundInitializer<SearchEngine>() {
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            @Override
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            protected SearchEngine initialize() throws Exception {
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                return createSearchEngine();
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            }
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        });
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        // Start all background tasks
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        MultiBackgroundInitializer.MultiBackgroundInitializerResults results = initializer.start();
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        // Get results (blocks until all complete)
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        DataSource dataSource = (DataSource) results.getInitializer("database").get();
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        CacheManager cacheManager = (CacheManager) results.getInitializer("cache").get();
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        SearchEngine searchEngine = (SearchEngine) results.getInitializer("search").get();
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        // Check for exceptions
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        if (results.isException("database")) {
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            Exception dbException = results.getException("database");
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            log.error("Database initialization failed", dbException);
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        }
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    }
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}
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```
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### Circuit Breaker Pattern
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#### EventCountCircuitBreaker - Failure Rate Protection
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```java { .api }
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import org.apache.commons.lang3.concurrent.EventCountCircuitBreaker;
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```
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```java { .api }
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public class CircuitBreakerExample {
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    // Circuit breaker: max 5 failures in 1 minute, then open for 30 seconds
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    private final EventCountCircuitBreaker circuitBreaker = 
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        new EventCountCircuitBreaker(5, 1, TimeUnit.MINUTES, 3, 30, TimeUnit.SECONDS);
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    public String callExternalService(String request) throws ServiceException {
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        // Check if circuit is open (too many recent failures)
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        if (!circuitBreaker.checkState()) {
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            throw new ServiceException("Circuit breaker is OPEN - service temporarily unavailable");
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        }
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        try {
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            // Attempt the potentially failing operation
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            String response = externalServiceClient.call(request);
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            // Reset failure count on success
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            circuitBreaker.close();
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            return response;
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        } catch (Exception e) {
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            // Record failure
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            circuitBreaker.incrementAndCheckState();
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            throw new ServiceException("External service call failed", e);
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        }
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    }
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    // Monitor circuit breaker state
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    public CircuitBreakerStatus getCircuitBreakerStatus() {
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        return new CircuitBreakerStatus(
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            circuitBreaker.getState().name(),
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            circuitBreaker.getCheckInterval(),
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            circuitBreaker.getCheckIntervalUnit(),
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            circuitBreaker.getOpeningThreshold(),
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            circuitBreaker.getClosingThreshold()
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        );
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    }
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}
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```
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#### ThresholdCircuitBreaker - Simple Threshold Protection
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```java { .api }
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import org.apache.commons.lang3.concurrent.ThresholdCircuitBreaker;
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```
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```java { .api }
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public class ThresholdCircuitBreakerExample {
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    // Circuit breaker that opens after 10 failures
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    private final ThresholdCircuitBreaker circuitBreaker = new ThresholdCircuitBreaker(10);
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    public void processMessage(Message message) throws ProcessingException {
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        if (!circuitBreaker.checkState()) {
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            throw new ProcessingException("Message processing circuit breaker is OPEN");
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        }
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        try {
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            messageProcessor.process(message);
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            // Reset on successful processing
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            // Note: ThresholdCircuitBreaker doesn't auto-reset, manual reset needed
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        } catch (Exception e) {
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            circuitBreaker.incrementAndCheckState();
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            throw new ProcessingException("Message processing failed", e);
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        }
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    }
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    // Manual reset method (could be called by admin endpoint)
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    public void resetCircuitBreaker() {
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        circuitBreaker.close();
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    }
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}
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```
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### Thread Factory and Management
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#### BasicThreadFactory - Customizable Thread Creation
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```java { .api }
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import org.apache.commons.lang3.concurrent.BasicThreadFactory;
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```
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```java { .api }
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public class ThreadFactoryExample {
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    // Custom thread factory with naming and daemon settings
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    private final ThreadFactory threadFactory = new BasicThreadFactory.Builder()
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        .namingPattern("worker-thread-%d")
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        .daemon(true)
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        .priority(Thread.NORM_PRIORITY)
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        .uncaughtExceptionHandler((thread, exception) -> {
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            log.error("Uncaught exception in thread {}: {}", thread.getName(), exception.getMessage(), exception);
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        })
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        .build();
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    private final ExecutorService executorService = Executors.newFixedThreadPool(10, threadFactory);
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    // Background task processing
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    public void submitBackgroundTask(Runnable task) {
380
        executorService.submit(() -> {
381
            try {
382
                task.run();
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            } catch (Exception e) {
384
                log.error("Background task failed", e);
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                // Exception is also handled by uncaught exception handler
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            }
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        });
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    }
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    // Scheduled task executor with custom threads
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    private final ScheduledExecutorService scheduledExecutor = 
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        Executors.newScheduledThreadPool(5, new BasicThreadFactory.Builder()
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            .namingPattern("scheduled-task-%d")
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            .daemon(false) // Keep JVM alive
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            .priority(Thread.MAX_PRIORITY)
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            .build());
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    public void scheduleMaintenanceTask() {
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        scheduledExecutor.scheduleAtFixedRate(() -> {
400
            performMaintenance();
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        }, 0, 1, TimeUnit.HOURS);
402
    }
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}
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```
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### Advanced Concurrent Patterns
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#### Memoizer - Thread-Safe Caching
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```java { .api }
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import org.apache.commons.lang3.concurrent.Memoizer;
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```
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```java { .api }
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public class MemoizerExample {
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417
    // Cache expensive computations
418
    private final Memoizer<String, UserProfile> userProfileCache = 
419
        new Memoizer<>(userId -> {
420
            // This computation happens only once per userId
421
            return loadUserProfileFromDatabase(userId);
422
        });
423
    
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    public UserProfile getUserProfile(String userId) throws InterruptedException, ExecutionException {
425
        return userProfileCache.compute(userId);
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    }
427
    
428
    // With custom cache implementation
429
    private final Memoizer<CacheKey, Report> reportCache = 
430
        new Memoizer<>(
431
            key -> generateReport(key),
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            new ConcurrentHashMap<>(), // Custom cache implementation
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            true // Recalculate on InterruptedException
434
        );
435
    
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    public Report getReport(String reportType, Date startDate, Date endDate) 
437
            throws InterruptedException, ExecutionException {
438
        CacheKey key = new CacheKey(reportType, startDate, endDate);
439
        return reportCache.compute(key);
440
    }
441
}
442
```
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#### TimedSemaphore - Rate Limiting
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```java { .api }
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import org.apache.commons.lang3.concurrent.TimedSemaphore;
448
```
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450
```java { .api }
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public class RateLimitingExample {
452
    
453
    // Allow maximum 100 operations per second
454
    private final TimedSemaphore rateLimiter = new TimedSemaphore(1, TimeUnit.SECONDS, 100);
455
    
456
    public void callRateLimitedAPI(String request) throws InterruptedException {
457
        // This will block if rate limit is exceeded
458
        rateLimiter.acquire();
459
        
460
        try {
461
            apiClient.makeCall(request);
462
        } finally {
463
            // Permits are automatically released after the time period
464
        }
465
    }
466
    
467
    // Dynamic rate adjustment
468
    public void adjustRateLimit(int newLimit) {
469
        rateLimiter.setLimit(newLimit);
470
    }
471
    
472
    // Monitor rate limiter
473
    public RateLimiterStatus getRateLimiterStatus() {
474
        return new RateLimiterStatus(
475
            rateLimiter.getLimit(),
476
            rateLimiter.getAvailablePermits(),
477
            rateLimiter.getAcquireCount(),
478
            rateLimiter.getPeriod(),
479
            rateLimiter.getUnit()
480
        );
481
    }
482
    
483
    // Shutdown
484
    public void shutdown() {
485
        rateLimiter.shutdown();
486
    }
487
}
488
```
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## Real-World Integration Examples
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492
### Spring Boot Integration
493

494
```java { .api }
495
@Configuration
496
@EnableAsync
497
public class ConcurrentConfiguration {
498
    
499
    @Bean
500
    public ThreadFactory taskThreadFactory() {
501
        return new BasicThreadFactory.Builder()
502
            .namingPattern("async-task-%d")
503
            .daemon(true)
504
            .priority(Thread.NORM_PRIORITY)
505
            .uncaughtExceptionHandler((thread, exception) -> {
506
                log.error("Uncaught exception in async task thread {}", thread.getName(), exception);
507
            })
508
            .build();
509
    }
510
    
511
    @Bean
512
    public TaskExecutor taskExecutor(ThreadFactory taskThreadFactory) {
513
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
514
        executor.setCorePoolSize(10);
515
        executor.setMaxPoolSize(20);
516
        executor.setQueueCapacity(100);
517
        executor.setThreadFactory(taskThreadFactory);
518
        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
519
        executor.initialize();
520
        return executor;
521
    }
522
    
523
    @Bean
524
    public EventCountCircuitBreaker externalServiceCircuitBreaker() {
525
        // 5 failures per minute opens circuit for 30 seconds
526
        return new EventCountCircuitBreaker(5, 1, TimeUnit.MINUTES, 3, 30, TimeUnit.SECONDS);
527
    }
528
}
529

530
@Service
531
public class ExternalServiceClient {
532
    
533
    private final EventCountCircuitBreaker circuitBreaker;
534
    private final RestTemplate restTemplate;
535
    
536
    public ExternalServiceClient(EventCountCircuitBreaker circuitBreaker, RestTemplate restTemplate) {
537
        this.circuitBreaker = circuitBreaker;
538
        this.restTemplate = restTemplate;
539
    }
540
    
541
    public CompletableFuture<String> callExternalServiceAsync(String request) {
542
        return CompletableFuture.supplyAsync(() -> {
543
            if (!circuitBreaker.checkState()) {
544
                throw new ServiceUnavailableException("External service circuit breaker is OPEN");
545
            }
546
            
547
            try {
548
                String response = restTemplate.getForObject("/external/api?q=" + request, String.class);
549
                circuitBreaker.close(); // Reset on success
550
                return response;
551
            } catch (Exception e) {
552
                circuitBreaker.incrementAndCheckState();
553
                throw new ExternalServiceException("External service call failed", e);
554
            }
555
        });
556
    }
557
}
558
```
559

560
### Microservice Resilience Pattern
561

562
```java { .api }
563
@Component
564
public class ResilientServiceClient {
565
    
566
    private final TimedSemaphore rateLimiter;
567
    private final EventCountCircuitBreaker circuitBreaker;
568
    private final Memoizer<String, ServiceResponse> cache;
569
    
570
    public ResilientServiceClient() {
571
        // Rate limiting: 50 requests per second
572
        this.rateLimiter = new TimedSemaphore(1, TimeUnit.SECONDS, 50);
573
        
574
        // Circuit breaker: 10 failures in 2 minutes opens for 60 seconds
575
        this.circuitBreaker = new EventCountCircuitBreaker(10, 2, TimeUnit.MINUTES, 5, 60, TimeUnit.SECONDS);
576
        
577
        // Response caching
578
        this.cache = new Memoizer<>(this::callServiceWithoutProtection);
579
    }
580
    
581
    public ServiceResponse callService(String request) throws ServiceException {
582
        try {
583
            // Apply rate limiting
584
            rateLimiter.acquire();
585
            
586
            // Check circuit breaker
587
            if (!circuitBreaker.checkState()) {
588
                return getFallbackResponse(request);
589
            }
590
            
591
            // Use cached response if available
592
            return cache.compute(request);
593
            
594
        } catch (InterruptedException e) {
595
            Thread.currentThread().interrupt();
596
            throw new ServiceException("Service call interrupted", e);
597
        } catch (ExecutionException e) {
598
            circuitBreaker.incrementAndCheckState();
599
            throw new ServiceException("Service call failed", e.getCause());
600
        }
601
    }
602
    
603
    private ServiceResponse callServiceWithoutProtection(String request) {
604
        try {
605
            ServiceResponse response = httpClient.call(request);
606
            circuitBreaker.close(); // Reset on success
607
            return response;
608
        } catch (Exception e) {
609
            throw new RuntimeException("Service call failed", e);
610
        }
611
    }
612
    
613
    private ServiceResponse getFallbackResponse(String request) {
614
        return ServiceResponse.fallback("Service temporarily unavailable");
615
    }
616
    
617
    @PreDestroy
618
    public void shutdown() {
619
        rateLimiter.shutdown();
620
    }
621
}
622
```
623

624
### Application Startup Coordinator
625

626
```java { .api }
627
@Component
628
public class ApplicationStartupCoordinator {
629
    
630
    private final MultiBackgroundInitializer startupInitializer;
631
    private volatile boolean applicationReady = false;
632
    
633
    @EventListener(ApplicationStartedEvent.class)
634
    public void onApplicationStarted() throws ConcurrentException {
635
        log.info("Starting background initialization...");
636
        
637
        MultiBackgroundInitializer initializer = new MultiBackgroundInitializer();
638
        
639
        // Database initialization
640
        initializer.addInitializer("database", new BackgroundInitializer<DataSource>() {
641
            @Override
642
            protected DataSource initialize() throws Exception {
643
                log.info("Initializing database connections...");
644
                return dataSourceFactory.createDataSource();
645
            }
646
        });
647
        
648
        // Cache warming
649
        initializer.addInitializer("cache", new BackgroundInitializer<CacheManager>() {
650
            @Override
651
            protected CacheManager initialize() throws Exception {
652
                log.info("Warming up caches...");
653
                CacheManager cacheManager = createCacheManager();
654
                cacheManager.preloadData();
655
                return cacheManager;
656
            }
657
        });
658
        
659
        // External service health check
660
        initializer.addInitializer("external-services", new BackgroundInitializer<HealthCheckResults>() {
661
            @Override
662
            protected HealthCheckResults initialize() throws Exception {
663
                log.info("Checking external service health...");
664
                return healthChecker.checkAllServices();
665
            }
666
        });
667
        
668
        // Start all background tasks
669
        MultiBackgroundInitializer.MultiBackgroundInitializerResults results = initializer.start();
670
        
671
        // Wait for completion and handle results
672
        handleInitializationResults(results);
673
        
674
        applicationReady = true;
675
        log.info("Application startup completed successfully");
676
    }
677
    
678
    private void handleInitializationResults(MultiBackgroundInitializer.MultiBackgroundInitializerResults results) {
679
        for (String taskName : Arrays.asList("database", "cache", "external-services")) {
680
            if (results.isException(taskName)) {
681
                Exception exception = results.getException(taskName);
682
                log.error("Initialization failed for {}: {}", taskName, exception.getMessage(), exception);
683
                
684
                // Decide whether to fail startup or continue
685
                if ("database".equals(taskName)) {
686
                    throw new ApplicationStartupException("Critical component failed: " + taskName, exception);
687
                } else {
688
                    log.warn("Non-critical component failed, continuing startup: {}", taskName);
689
                }
690
            } else {
691
                log.info("Successfully initialized: {}", taskName);
692
            }
693
        }
694
    }
695
    
696
    @EventListener(ContextClosedEvent.class)
697
    public void onApplicationShutdown() {
698
        if (startupInitializer != null) {
699
            // Graceful shutdown of background tasks
700
            try {
701
                startupInitializer.shutdown();
702
            } catch (Exception e) {
703
                log.warn("Error during shutdown: {}", e.getMessage(), e);
704
            }
705
        }
706
    }
707
    
708
    public boolean isApplicationReady() {
709
        return applicationReady;
710
    }
711
}
712
```
713

714
The concurrent utilities in Apache Commons Lang provide essential building blocks for creating robust, thread-safe applications with proper initialization patterns, fault tolerance, and resource management that scale well under concurrent load.