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1. Introduction

When we want to copy an object in Java, there’re two possibilities that we need to consider — a shallow copy and a deep copy.

The shallow copy is the approach when we only copy field values and therefore the copy might be dependant on the original object. In the deep copy approach, we make sure that all the objects in the tree are deeply copied, so the copy isn’t dependant on any earlier existing object that might ever change.

In this article, we’ll compare these two approaches and learn four methods to implement the deep copy.

2. Maven Setup

We’ll use three Maven dependencies — Gson, Jackson, and Apache Commons Lang — to test different ways of performing a deep copy.

Let’s add these dependencies to our pom.xml:

<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.8.2</version>
</dependency>
<dependency>
    <groupId>commons-lang</groupId>
    <artifactId>commons-lang</artifactId>
    <version>2.6</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.9.3</version>
</dependency>

The latest versions of Gson, Jackson, and Apache Commons Lang can be found on Maven Central.

3. Model

To compare different methods to copy Java objects, we’ll need two classes to work on:

class Address {

    private String street;
    private String city;
    private String country;

    // standard constructors, getters and setters
}
class User {

    private String firstName;
    private String lastName;
    private Address address;

    // standard constructors, getters and setters
}

4. Shallow Copy

A shallow copy is one in which we only copy values of fields from one object to another:

@Test
public void whenShallowCopying_thenObjectsShouldNotBeSame() {

    Address address = new Address("Downing St 10", "London", "England");
    User pm = new User("Prime", "Minister", address);
    
    User shallowCopy = new User(
      pm.getFirstName(), pm.getLastName(), pm.getAddress());

    assertThat(shallowCopy)
      .isNotSameAs(pm);
}

In this case pm != shallowCopy, which means that they’re different objects, but the problem is that when we change any of the original address’ properties, this will also affect the shallowCopy‘s address.

We wouldn’t bother about it if Address was immutable, but it’s not:

@Test
public void whenModifyingOriginalObject_ThenCopyShouldChange() {
 
    Address address = new Address("Downing St 10", "London", "England");
    User pm = new User("Prime", "Minister", address);
    User shallowCopy = new User(
      pm.getFirstName(), pm.getLastName(), pm.getAddress());

    address.setCountry("Great Britain");
    assertThat(shallowCopy.getAddress().getCountry())
      .isEqualTo(pm.getAddress().getCountry());
}

5. Deep Copy

A deep copy is an alternative that solves this problem. Its advantage is that at least each mutable object in the object graph is recursively copied.

Since the copy isn’t dependent on any mutable object that was created earlier, it won’t get modified by accident like we saw with the shallow copy.

In the following sections, we’ll show several deep copy implementations and demonstrate this advantage.

5.1. Copy Constructor

The first implementation we’ll implement is based on copy constructors:

public Address(Address that) {
    this(that.getStreet(), that.getCity(), that.getCountry());
}
public User(User that) {
    this(that.getFirstName(), that.getLastName(), new Address(that.getAddress()));
}

In the above implementation of the deep copy, we haven’t created new Strings in our copy constructor because String is an immutable class.

As a result, they can’t be modified by accident. Let’s see if this works:

@Test
public void whenModifyingOriginalObject_thenCopyShouldNotChange() {
    Address address = new Address("Downing St 10", "London", "England");
    User pm = new User("Prime", "Minister", address);
    User deepCopy = new User(pm);

    address.setCountry("Great Britain");
    assertNotEquals(
      pm.getAddress().getCountry(), 
      deepCopy.getAddress().getCountry());
}

5.2. Cloneable Interface

The second implementation is based on the clone method inherited from Object. It’s protected, but we need to override it as public.

We’ll also add a marker interface, Cloneable, to the classes to indicate that the classes are actually cloneable.

Let’s add the clone() method to the Address class:

@Override
public Object clone() {
    try {
        return (Address) super.clone();
    } catch (CloneNotSupportedException e) {
        return new Address(this.street, this.getCity(), this.getCountry());
    }
}

And now let’s implement clone() for the User class:

@Override
public Object clone() {
    User user = null;
    try {
        user = (User) super.clone();
    } catch (CloneNotSupportedException e) {
        user = new User(
          this.getFirstName(), this.getLastName(), this.getAddress());
    }
    user.address = (Address) this.address.clone();
    return user;
}

Note that the super.clone() call returns a shallow copy of an object, but we set deep copies of mutable fields manually, so the result is correct:

@Test
public void whenModifyingOriginalObject_thenCloneCopyShouldNotChange() {
    Address address = new Address("Downing St 10", "London", "England");
    User pm = new User("Prime", "Minister", address);
    User deepCopy = (User) pm.clone();

    address.setCountry("Great Britain");

    assertThat(deepCopy.getAddress().getCountry())
      .isNotEqualTo(pm.getAddress().getCountry());
}

6. External Libraries

The above examples look easy, but sometimes they don’t apply as a solution when we can’t add an additional constructor or override the clone method.

This might happen when we don’t own the code, or when the object graph is so complicated that we wouldn’t finish our project on time if we focused on writing additional constructors or implementing the clone method on all classes in the object graph.

What then? In this case, we can use an external library. To achieve a deep copy, we can serialize an object and then deserialize it to a new object.

Let’s look at a few examples.

6.1. Apache Commons Lang

Apache Commons Lang has SerializationUtils#clone, which performs a deep copy when all classes in the object graph implement the Serializable interface.

If the method encounters a class that isn’t serializable, it’ll fail and throw an unchecked SerializationException.

Because of that, we need to add the Serializable interface to our classes:

@Test
public void whenModifyingOriginalObject_thenCommonsCloneShouldNotChange() {
    Address address = new Address("Downing St 10", "London", "England");
    User pm = new User("Prime", "Minister", address);
    User deepCopy = (User) SerializationUtils.clone(pm);

    address.setCountry("Great Britain");

    assertThat(deepCopy.getAddress().getCountry())
      .isNotEqualTo(pm.getAddress().getCountry());
}

6.2. JSON Serialization with Gson

The other way to serialize is to use JSON serialization. Gson is a library that’s used for converting objects into JSON and vice versa.

Unlike Apache Commons Lang, GSON does not need the Serializable interface to make the conversions.

Let’s have a quick look at an example:

@Test
public void whenModifyingOriginalObject_thenGsonCloneShouldNotChange() {
    Address address = new Address("Downing St 10", "London", "England");
    User pm = new User("Prime", "Minister", address);
    Gson gson = new Gson();
    User deepCopy = gson.fromJson(gson.toJson(pm), User.class);

    address.setCountry("Great Britain");

    assertThat(deepCopy.getAddress().getCountry())
      .isNotEqualTo(pm.getAddress().getCountry());
}

6.3. JSON Serialization with Jackson

Jackson is another library that supports JSON serialization. This implementation will be very similar to the one using Gson, but we need to add the default constructor to our classes.

Let’s see an example:

@Test
public void whenModifyingOriginalObject_thenJacksonCopyShouldNotChange() throws IOException {
    Address address = new Address("Downing St 10", "London", "England");
    User pm = new User("Prime", "Minister", address);
    ObjectMapper objectMapper = new ObjectMapper();
    
    User deepCopy = objectMapper
      .readValue(objectMapper.writeValueAsString(pm), User.class);

    address.setCountry("Great Britain");

    assertThat(deepCopy.getAddress().getCountry())
      .isNotEqualTo(pm.getAddress().getCountry());
}

7. Conclusion

Which implementation should we use when making a deep copy? The final decision will often depend on the classes we’ll copy and whether we own the classes in the object graph.

As always, the complete code samples for this tutorial can be found over on Github.

I just announced the new Spring 5 modules in REST With Spring:

>> CHECK OUT THE LESSONS