Chain of Responsibility and Pipelines

When processing requires multiple steps or multiple handlers might process a request, Chain of Responsibility and Pipeline patterns provide clean solutions. This article covers classic chains, middleware patterns, and type-safe pipelines.

📋 At a Glance

Aspect	Details
Patterns Covered	Chain of Responsibility, Pipeline, Middleware
Primary Benefit	Decoupled processing, easy to extend
Use Cases	Validation, request handling, data transformation
Java Features	Lambdas, Generics, Records

🎯 What You'll Learn

Chain of Responsibility for handler delegation
Pipeline Pattern for sequential transformations
Middleware Pattern (request/response chains)
Spring integration with interceptors and filters
When to use chains vs simple conditionals

Production Story: The Validation Spaghetti

An e-commerce checkout had validation scattered everywhere:

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The fix: Validation chain:

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Mental Model: Chain vs Pipeline

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🔬 Deep Dive

Pattern 1: Classic Chain of Responsibility

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Pattern 2: Functional Chain (Modern Approach)

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Pattern 3: Validation Chain

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Pattern 4: Pipeline Pattern

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Pipeline with error handling:

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Pattern 5: Middleware Pattern

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Pattern 6: Spring Integration

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⚠️ Common Mistakes

Mistake 1: Missing Termination

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Mistake 2: Tight Coupling in Chain

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🐛 Debug This: The Broken Chain

A developer reports: "Our validation chain works for some requests but silently skips validation for others!"

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Why does the email validation get skipped?

✅ Solution:

The bug is in RequiredFieldsValidator - when validation passes (name is not null), it returns ValidationResult.ok() without calling passToNext(). This breaks the chain completely.

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The fix:

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Better approach - use Template Method to prevent this bug:

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The lesson: When implementing Chain of Responsibility, ensure chain continuation is enforced by the base class, not left to subclass implementations.

💻 Exercises

Exercise 1: Simple Handler Chain

⭐ Difficulty: Easy | ⏱️ Time: 15 minutes

Task: Implement a discount handler chain for e-commerce.

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✅ Solution:

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Exercise 2: Validation Pipeline

⭐⭐ Difficulty: Medium | ⏱️ Time: 20 minutes

Task: Create a validation pipeline that collects all errors (not fail-fast).

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✅ Solution:

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Exercise 3: Middleware Chain

⭐⭐ Difficulty: Medium | ⏱️ Time: 25 minutes

Task: Implement HTTP middleware pattern with before/after hooks.

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✅ Solution:

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Exercise 4: Type-Safe Pipeline

⭐⭐⭐ Difficulty: Medium-Hard | ⏱️ Time: 25 minutes

Task: Create a type-safe pipeline where each step transforms the type.

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✅ Solution:

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Exercise 5: Pipeline with Error Handling

⭐⭐⭐⭐ Difficulty: Hard | ⏱️ Time: 30 minutes

Task: Create a pipeline with Result type for error propagation.

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✅ Solution:

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🎤 Interview Questions

Q1: When use Chain of Responsibility vs simple if-else?

Answer: Use Chain when:

Handlers are independent and reusable
Need to add/remove handlers at runtime
Different configurations need different chains
Handlers are in different modules/teams

Use if-else when:

Logic is simple (2-3 cases)
Conditions are tightly coupled
No reuse needed

Q2: Difference between Chain of Responsibility and Pipeline?

Answer:

Chain: Handler decides if it should process; request may be handled by 0, 1, or many handlers
Pipeline: All steps execute in sequence; transforms data step by step

Chain = "who should handle this?" Pipeline = "process this through all steps"

📝 Summary

Pattern	Use When
Chain of Responsibility	Multiple potential handlers, one processes
Pipeline	Sequential transformations
Middleware	Request/response processing
Validation Chain	Multiple validators, error accumulation
Spring Interceptors	HTTP request processing

📅 Review Schedule for This Article

Day	Task	Time
Day 1	Review Chain vs Pipeline mental model	5 min
Day 3	Redo Exercise 2 (Validation Pipeline)	20 min
Day 7	Answer interview questions without looking	10 min
Day 14	Redo Debug This (Broken Chain)	15 min
Day 30	Identify one chain/pipeline opportunity in your codebase	20 min

Next: [Part 13: Template Method & Hooks]

Chain of Responsibility and Pipelines

📋 At a Glance

🎯 What You'll Learn

Production Story: The Validation Spaghetti

Mental Model: Chain vs Pipeline

🔬 Deep Dive

Pattern 1: Classic Chain of Responsibility

Pattern 2: Functional Chain (Modern Approach)

Pattern 3: Validation Chain

Pattern 4: Pipeline Pattern

Pattern 5: Middleware Pattern

Pattern 6: Spring Integration

⚠️ Common Mistakes

Mistake 1: Missing Termination

Mistake 2: Tight Coupling in Chain

🐛 Debug This: The Broken Chain

💻 Exercises

Exercise 1: Simple Handler Chain

Exercise 2: Validation Pipeline

Exercise 3: Middleware Chain

Exercise 4: Type-Safe Pipeline

Exercise 5: Pipeline with Error Handling

🎤 Interview Questions

Q1: When use Chain of Responsibility vs simple if-else?

Q2: Difference between Chain of Responsibility and Pipeline?

📝 Summary

📅 Review Schedule for This Article

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