---
name: understand-code
description: Help understand decompiled Java code, analyze business logic, and document code functionality. Use when user needs to comprehend code purpose, flow, or behavior.
version: 1.0.0
---

# Code Understanding Guide

This skill helps understand decompiled or existing Java code.

## When to Use

- User has decompiled code and needs to understand it
- User wants to document business logic
- User needs to trace code flow
- User wants to identify code patterns

## Analysis Approach

### 1. Start with Entry Points

Identify how the application is accessed:

```bash
# Find Servlet/Action classes
find <dir> -name "*Servlet.java" -o -name "*Action.java" | head -20

# Find Controller classes
find <dir> -name "*Controller.java" | head -20

# Find Service endpoints
find <dir> -name "*Service.java" -o -name "*ServiceImpl.java" | head -20
```

### 2. Trace Request Flow

For Struts applications:

```
HTTP Request
    ↓
Struts Filter (web.xml)
    ↓
Action Mapping (struts.xml)
    ↓
Action Class (execute method)
    ↓
Service Layer
    ↓
DAO Layer
    ↓
Database
```

For Spring applications:

```
HTTP Request
    ↓
DispatcherServlet
    ↓
Controller (@RequestMapping)
    ↓
Service Layer (@Service)
    ↓
Repository (@Repository)
    ↓
Database
```

### 3. Understand Domain Model

Analyze domain classes:

```bash
# Find domain/entity classes
find <dir> -name "*Domain.java" -o -name "*Entity.java" -o -name "*Bean.java"

# Check for common patterns
grep -l "get\|set" <dir>/*Domain.java | head -10
```

Domain class patterns:
- Fields with getters/setters
- Serializable implementation
- JPA/Hibernate annotations
- Relationships (OneToMany, ManyToOne)

### 4. Map Business Logic

Identify business operations:

```bash
# Find service methods
grep -r "public.*Service" <dir> | head -20

# Find transaction boundaries
grep -r "@Transactional\|beginTransaction" <dir>

# Find business calculations
grep -r "calculate\|compute\|process" <dir>
```

### 5. Configuration Analysis

Read configuration files:

```bash
# Spring context
cat <dir>/spring-application-context.xml

# Struts config
cat <dir>/struts.xml

# Database config
cat <dir>/database.properties

# Web config
cat <dir>/web.xml
```

## Code Patterns

### DAO Pattern
```java
public interface UserDao {
    User findById(Long id);
    List<User> findAll();
    void save(User user);
    void delete(User user);
}

public class UserDaoImpl implements UserDao {
    // Implementation using iBatis/Hibernate
}
```

### Service Pattern
```java
public interface UserService {
    User getUser(Long id);
    void createUser(User user);
}

public class UserServiceImpl implements UserService {
    private UserDao userDao;
    
    @Transactional
    public void createUser(User user) {
        // Business logic
        userDao.save(user);
    }
}
```

### Action Pattern (Struts)
```java
public class UserAction extends ActionSupport {
    private UserService userService;
    private User user;
    
    public String execute() {
        // Handle request
        user = userService.getUser(id);
        return SUCCESS;
    }
}
```

## Documentation Generation

Generate code documentation:

### 1. Class Summary

For each class, document:
- Purpose/responsibility
- Key methods
- Dependencies
- Usage examples

### 2. Flow Diagrams

Create text-based flow diagrams:

```
User Login Flow:
┌─────────┐    ┌─────────┐    ┌─────────┐
│  Login  │───→│ Action  │───→│ Service │
│   Page  │    │         │    │         │
└─────────┘    └─────────┘    └─────────┘
                    │              │
                    ↓              ↓
               ┌─────────┐    ┌─────────┐
               │ Validate │    │   DAO   │
               │  Input   │    │         │
               └─────────┘    └─────────┘
```

### 3. Package Documentation

Document each package:

```
com.hst.taxService.action
- Purpose: Struts actions for tax service web interface
- Key classes: TaxAction, ReportAction
- Dependencies: taxService.service, common.util
```

## Analysis Tools

### Quick Analysis

```bash
# Count methods per class
for f in $(find <dir> -name "*.java"); do
    echo "$(grep -c "public\|private\|protected" "$f") $f"
done | sort -rn | head -20

# Find complex methods (many lines)
for f in $(find <dir> -name "*.java"); do
    echo "$(wc -l < "$f") $f"
done | sort -rn | head -20
```

### Pattern Detection

```bash
# Find singletons
grep -r "getInstance\|private static" <dir>

# Find factories
grep -r "Factory\|createInstance" <dir>

# Find observers
grep -r "Listener\|Observer\|addEventListener" <dir>
```

## Integration

After understanding code:

1. Create README documentation
2. Generate API documentation
3. Identify refactoring opportunities
4. Plan modernization if needed