How to Write Unit Test for

New Code on top of Legacy Code

Joseph Yao

Please Read This Great Book!

Unit Test 101

What’s Unit Test?

Unit tests is the idea that they are tests in

isolation of individual components of software

- Michael C. Feathers

What’s Legacy Code?

Legacy code is simply code without tests.

Without tests is bad code. It doesn't matter how well written

it is; it doesn't matter how pretty or object-oriented or well-

encapsulated it is. With tests, we can change the behavior

of our code quickly and verifiably. Without them, we really

don't know if our code is getting better or worse.

- Michael C. Feathers

Make New Code Testable

How to Initialize Dependency

Need

Legacy Code

New Code

Your Code Looks Like This? public class Car {

private Engine engine;

public Car() {

engine = new Engine();

}

public void run() {

engine.start();

}

public String status() {

return engine.speed() > 0 ? “Move”: “Stop”;

}

}

Your Test Looks Like This?

Are you really do the unit test for Car?

public class TestCar {

@Test public void move() {

Car car = new Car();

car.run();

assertEquals(“Move”, car.status());

}

}

Code for Car with Isolation public class Car {

private IEngine engine;

public Car(IEngine theEngine) {

engine = theEngine;

}

public void run() {

engine.start();

}

public String status() {

return engine.speed() > 0 ? “Move”: “Stop”;

}

}

Test for Car with Isolation public class TestCar {

@Test public void move() {

IEngine engineMock =

new EngineMock(10);

Car car = new Car(engineMock);

car.run();

assertEquals(“Move”, car.status());

}

}

In fact, you don’t care how engine speed is calculated

I Need to Test a Private Method

Your Code Looks Like This?

How can I test the private method?

public class Order {

public void addItem(Item item) {

if (isValidItem(item)) {

items.add(item);

}

}

private boolean isValidItem(Item item) {

more than 500 lines code…

}

}

Does Your Class Have too many Responsibilities?

Maybe This Code is Better? public class Order {

private ItemValidator validator;

public Order (ItemValidator

theValidator) {

validator = theValidator;

}

public void addItem (Item item) {

if (validator.

isValidItem(item)) {

…

}

}

}

Single Responsibility Principle

“Don’t Do it” unless you have a Very Strong Reason

+

Good design is testable, and

design that isn't testable is bad

- Michael C. Feathers

Isolate Legacy Code

Too hard to get Legacy Code under Test

Some New Code needed to be added

public class Customer {

…

public void purchase() {

more than 500 lines of legacy code…

}

…

}

We need to log this customer purchase action after it done.

Can you add unit test for new code with no impact to legacy code?

Add new code by Sprout Method

public class Customer {

public void purchase() {

purchaseWithoutLog();

logPurchaseAction();

}

protected void purchaseWithoutLog() {

more than 500 lines of legacy code…

}

private void logPurchaseAction() {

Your new code here…

}

}

Your Test May Look Like This public class TestCustomerPurchaseLog extends

Customer {

@Test public void log() {

Customer customer =

new TestCustomerPurchaseLog();

customer.purchase();

… code to verify the log action …

}

protected void purchaseWithoutLog() {}

}

Some other Alternative Ways

• Sprout Method – the Sample Code

• Wrap Method

• If the legacy class is hard to be put into test

harness

o Sprout Class

o Wrap Class

o This is quite useful when you can’t easily isolate the

dependency for legacy class

I have a Monster Dependency to Isolate

How to Isolate “HttpServletRequest”?

public class ARMDispatcher {

public void populate (HttpServletRequest request) {

String [] values

= request.getParameterValues(pageStateName);

if (values != null && values.length > 0) {

marketBindings.put(

pageStateName + getDateStamp(), values[0]);

}

}

}

You only Need to get the Parameter Value

public class ARMDispatcher {

public void populate (ParameterSource source) {

String value =

source.getParameterForName(pageStateName);

if (value != null) {

marketBindings.put(

ageStateName + getDateStamp(), value);

}

}

}

I need D, but it’s from A.getB().getC().getD()

Your Code may Look like this public class Customer {

private Orders orders;

public List<String> getAllItemNamesOfLatestOrder(){

List<String> allNames =

new ArrayList<String>();

Item[] items =

orders.getLatestOrder().getAllItems();

for (Item item : items) {

allNames.add(item.getName());

}

return allNames;

}

}

You need to Isolate the way to Get Items

public class Customer {

private Orders orders;

public List<String> getAllItemNamesOfLatestOrder(){

List<String> allNames =

new ArrayList<String>();

Item[] items = getAllItemsOfLatestOrder();

for (Item item : items) {

allNames.add(item.getName());

}

return allNames;

}

protected Item[] getAllItemsOfLatestOrder() {

return orders.getLatestOrder().getAllItems();

}

}

Suggestions

Test Behavior

instead of

Test Implementation

Dependency Isolation Tool is

Evil

Q & A

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