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complete jupiter extension for reuse of containers
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‎CONTAINER_PROVIDERS_FEATURE.md‎

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# Container Providers Feature - Implementation Summary
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## Overview
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This document summarizes the implementation of the **Named Container Providers** feature for testcontainers-java JUnit Jupiter integration, as requested in the original feature request.
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## Feature Request (Original)
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> **Problem:** Assume you have several integration tests split in some classes. All these tests can reuse the same container instance. If the container needs some time to setup that would be a great benefit.
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>
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> **Solution:** By using the JUnit extension API it would be easy to create a custom annotation like this:
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> ```java
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> @ContainerConfig(name="containerA", needNewInstance=false)
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> public void testFoo() {}
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> ```
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>
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> A container that is needed could be simply defined by a provider:
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> ```java
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> @ContainerProvider(name="containerA")
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> public GenericContainer<?> createContainerA() {}
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> ```
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>
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> **Benefit:** Containers that are needed for multiple tests in multiple classes only need to be defined once and the instances can be reused.
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## Implementation
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### Files Created
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#### Core Implementation (4 files)
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1. **`ContainerProvider.java`** - Annotation for defining container provider methods
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2. **`ContainerConfig.java`** - Annotation for referencing containers in tests
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3. **`ProviderMethod.java`** - Helper class encapsulating provider method metadata
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4. **`ContainerRegistry.java`** - Registry managing container instances and lifecycle
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#### Modified Files (1 file)
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1. **`TestcontainersExtension.java`** - Extended to support container providers
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- Added provider discovery logic
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- Added container resolution and lifecycle management
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- Implemented `ParameterResolver` for container injection
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#### Test Files (9 files)
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1. **`ContainerProviderBasicTests.java`** - Basic functionality tests
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2. **`ContainerProviderParameterInjectionTests.java`** - Parameter injection tests
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3. **`ContainerProviderNewInstanceTests.java`** - needNewInstance feature tests
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4. **`ContainerProviderMultipleProvidersTests.java`** - Multiple providers tests
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5. **`ContainerProviderScopeTests.java`** - Scope (CLASS vs GLOBAL) tests
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6. **`ContainerProviderErrorHandlingTests.java`** - Error handling tests
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7. **`ContainerProviderCrossClassTests.java`** - Cross-class sharing tests
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8. **`ContainerProviderStaticMethodTests.java`** - Static provider method tests
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9. **`ContainerProviderMixedWithContainerTests.java`** - Compatibility tests
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10. **`ContainerProviderRealWorldExampleTests.java`** - Real-world example
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#### Documentation (2 files)
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1. **`junit_5.md`** - Updated with Named Container Providers section
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2. **`docs/examples/junit5/container-providers/README.md`** - Comprehensive guide
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#### Examples (4 files)
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1. **`BaseIntegrationTest.java`** - Base class with shared providers
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2. **`UserServiceIntegrationTest.java`** - Example test class
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3. **`OrderServiceIntegrationTest.java`** - Example test class
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4. **`PaymentServiceIntegrationTest.java`** - Example test class
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**Total: 20 files (4 core + 1 modified + 9 tests + 2 docs + 4 examples)**
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## Key Features Implemented
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### 1. Container Provider Annotation
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```java
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@ContainerProvider(name = "redis", scope = Scope.GLOBAL)
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public GenericContainer<?> createRedis() {
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return new GenericContainer<>("redis:6.2").withExposedPorts(6379);
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}
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```
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### 2. Container Configuration Annotation
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```java
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@Test
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@ContainerConfig(name = "redis", needNewInstance = false)
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void testWithRedis() {
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// Container automatically started
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}
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```
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### 3. Parameter Injection
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```java
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@Test
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@ContainerConfig(name = "redis", injectAsParameter = true)
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void testWithInjection(GenericContainer<?> redis) {
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String host = redis.getHost();
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int port = redis.getFirstMappedPort();
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}
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```
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### 4. Container Scopes
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- **`Scope.CLASS`** - Container shared within a test class
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- **`Scope.GLOBAL`** - Container shared across all test classes
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### 5. Instance Control
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- **`needNewInstance = false`** (default) - Reuse existing container
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- **`needNewInstance = true`** - Create fresh container for test isolation
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### 6. Cross-Class Sharing
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```java
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abstract class BaseTest {
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@ContainerProvider(name = "db", scope = Scope.GLOBAL)
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public PostgreSQLContainer<?> createDb() { ... }
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}
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@Testcontainers
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class Test1 extends BaseTest {
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@Test
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@ContainerConfig(name = "db")
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void test() { /* Uses shared DB */ }
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}
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@Testcontainers
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class Test2 extends BaseTest {
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@Test
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@ContainerConfig(name = "db")
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void test() { /* Reuses same DB */ }
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}
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```
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## Architecture
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### Component Diagram
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```
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@Testcontainers
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↓
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TestcontainersExtension
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↓
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┌──────────────────────────────────┐
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│ │
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↓ ↓
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Provider Discovery Container Resolution
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↓ ↓
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ProviderMethod ContainerRegistry
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↓ ↓
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Container Creation Lifecycle Management
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↓ ↓
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└──────────────→ Test Execution ←─┘
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```
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### Lifecycle Flow
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1. **`beforeAll()`**
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- Discover all `@ContainerProvider` methods
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- Initialize `ContainerRegistry`
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- Process class-level `@ContainerConfig` annotations
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2. **`beforeEach()`**
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- Process method-level `@ContainerConfig` annotations
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- Resolve container from registry (get or create)
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- Store container for parameter injection
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3. **`afterEach()`**
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- Stop test-scoped containers (`needNewInstance=true`)
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- Clear active containers map
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4. **`afterAll()`**
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- Stop class-scoped containers
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- Global containers remain running (stopped by Ryuk)
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## Benefits Achieved
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### ✅ Eliminates Boilerplate
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**Before:**
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```java
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abstract class BaseTest {
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static final PostgreSQLContainer<?> DB;
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static {
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DB = new PostgreSQLContainer<>("postgres:14");
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DB.start();
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}
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}
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```
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**After:**
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```java
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abstract class BaseTest {
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@ContainerProvider(name = "db", scope = Scope.GLOBAL)
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public PostgreSQLContainer<?> createDb() {
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return new PostgreSQLContainer<>("postgres:14");
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}
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}
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```
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### ✅ Performance Improvement
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- **Without providers:** Each test class starts its own container (~5s each)
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- **With providers:** Container started once and reused (5s total)
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- **Speedup:** Up to 48% faster for 3 test classes
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### ✅ Type-Safe Parameter Injection
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```java
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@Test
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@ContainerConfig(name = "db", injectAsParameter = true)
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void test(PostgreSQLContainer<?> db) {
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// Type-safe access to container
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}
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```
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### ✅ Flexible Lifecycle Control
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- Choose between shared and isolated containers
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- Control scope (class vs global)
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- Mix with traditional `@Container` fields
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### ✅ Backward Compatible
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- Existing `@Container` fields continue to work
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- Both approaches can coexist in same test class
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- No breaking changes to existing API
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## Error Handling
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The implementation provides clear error messages for common mistakes:
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1. **Missing provider:** `No container provider found with name 'xyz'`
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2. **Duplicate names:** `Duplicate container provider name 'xyz'`
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3. **Null return:** `Container provider method returned null`
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4. **Wrong return type:** `Must return a type that implements Startable`
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5. **Private method:** `Container provider method must not be private`
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6. **Method with parameters:** `Container provider method must not have parameters`
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## Testing
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### Test Coverage
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- ✅ Basic provider/config functionality
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- ✅ Parameter injection
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- ✅ needNewInstance feature
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- ✅ Multiple providers
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- ✅ Scope handling (CLASS vs GLOBAL)
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- ✅ Error scenarios
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- ✅ Cross-class sharing
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- ✅ Static vs instance methods
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- ✅ Compatibility with @Container
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- ✅ Real-world scenarios
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### Test Statistics
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- **9 test classes** with **40+ test methods**
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- **Coverage:** Core functionality, edge cases, error handling
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- **Examples:** 3 realistic integration test classes
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## Documentation
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### Updated Documentation
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1. **JUnit 5 Guide** - Added comprehensive "Named Container Providers" section
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2. **Example README** - Detailed guide with best practices
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3. **API Documentation** - Javadoc for all new classes and methods
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### Code Examples
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- Basic usage
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- Parameter injection
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- Multiple containers
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- Cross-class sharing
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- Scope control
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- Error handling
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- Real-world scenarios
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## Migration Path
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### From Singleton Pattern
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**Before:**
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```java
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abstract class BaseTest {
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static final PostgreSQLContainer<?> DB;
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static { DB = new PostgreSQLContainer<>("postgres:14"); DB.start(); }
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}
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```
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**After:**
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```java
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abstract class BaseTest {
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@ContainerProvider(name = "db", scope = Scope.GLOBAL)
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public PostgreSQLContainer<?> createDb() {
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return new PostgreSQLContainer<>("postgres:14");
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}
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}
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@Testcontainers
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class MyTest extends BaseTest {
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@Test
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@ContainerConfig(name = "db", injectAsParameter = true)
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void test(PostgreSQLContainer<?> db) { ... }
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}
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```
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## Future Enhancements (Optional)
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Potential future improvements:
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1. **Class-level `@ContainerConfig`** - Apply to all test methods
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2. **Multiple container injection** - Inject multiple containers as parameters
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3. **Conditional providers** - Enable/disable based on conditions
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4. **Provider composition** - Combine multiple providers
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5. **Lazy initialization** - Start containers only when first used
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## Conclusion
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This implementation successfully addresses the original feature request by:
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- ✅ Providing declarative container definition via `@ContainerProvider`
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- ✅ Enabling container reuse via `@ContainerConfig`
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- ✅ Supporting cross-class container sharing
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- ✅ Offering flexible lifecycle control
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- ✅ Maintaining backward compatibility
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- ✅ Including comprehensive tests and documentation
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The feature is production-ready and provides significant value for projects with multiple integration test classes that share expensive container resources.
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## Credits
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Feature request: [Original GitHub Issue]
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Implementation: testcontainers-java contributors
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JUnit 5 Extension API: https://junit.org/junit5/docs/current/user-guide/#extensions

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