Moi je pense à l'enfant, Entouré de soldats, Moi je pense à l'enfant, Qui demande pourquoi (Non Non Rien N'a Changé - Les Poppys)

After 8 years developing server and embedded applications using Hibernate as ORM, squeezing my brain seeking solutions to improve Hibernate performance, reading blogs and attending conferences, I decided to share this knowledge acquired during these years with you.

This is the first post of many more posts to come:

Last year I went to Devoxx as speaker but also I attended Patrycja Wegrzynowicz conference about Hibernate Anti-Patterns. In that presentation Patrycja shows us an anti-pattern that shocks me because it proved to expect the unexpected.

We are going to see the effect it has when Hibernate detects a dirty collection and should re-create it.

Let's start with the model we are going to use, only two classes related with one-to-many association:

	@Entity
	public class Starship {
	private Long id;
	@Id @GeneratedValue(strategy=GenerationType.SEQUENCE) public Long getId() {return id;}
	public void setId(Long id) {this.id = id;}
	private Date launched;
	@Temporal(TemporalType.DATE) public Date getLaunched() {return launched;}
	public void setLaunched(Date launched) {this.launched = launched;}
	private String registry;
	@Column(unique=true, nullable=false) public String getRegistry() {return registry;}
	public void setRegistry(String registry) {this.registry = registry;}
	private StarshipClassEnum starshipClassEnum;
	@Enumerated public StarshipClassEnum getStarshipClassEnum() {return starshipClassEnum;}
	public void setStarshipClassEnum(StarshipClassEnum starshipClassEnum) {this.starshipClassEnum = starshipClassEnum;}
	private AffiliationEnum affiliationEnum;
	@Enumerated public AffiliationEnum getAffiliationEnum() {return affiliationEnum;}
	public void setAffiliationEnum(AffiliationEnum affiliationEnum) {this.affiliationEnum = affiliationEnum;}
	private Physics physics;
	@Embedded public Physics getPhysics() {return physics;}
	public void setPhysics(Physics physics) {this.physics = physics;}
	private List<Officer> officers = new ArrayList<Officer>();
	@OneToMany(cascade={CascadeType.ALL}) public List<Officer> getOfficers() {return Collections.unmodifiableList(officers);}
	protected void setOfficers(List<Officer> officers) {this.officers = officers;}
	public void addOfficer(Officer officer) {
	officer.setStarship(this);
	this.officers.add(officer);
	}
	public Starship() {
	super();
	}
	public Starship(String registry) {
	setRegistry(registry);
	}
	@Override
	public int hashCode() {
	final int prime = 31;
	int result = 1;
	result = prime * result
	+ ((registry == null) ? 0 : registry.hashCode());
	return result;
	}
	@Override
	public boolean equals(Object obj) {
	if (this == obj)
	return true;
	if (obj == null)
	return false;
	if (getClass() != obj.getClass())
	return false;
	Starship other = (Starship) obj;
	if (registry == null) {
	if (other.registry != null)
	return false;
	} else if (!registry.equals(other.registry))
	return false;
	return true;
	}
	}

view raw Starship.java hosted with ❤ by GitHub

	@Entity
	public class Officer {
	private Long id;
	@Id @GeneratedValue(strategy=GenerationType.SEQUENCE)public Long getId() {return id;}
	protected void setId(Long id) {this.id = id;}
	private String name;
	@Column(unique=true, nullable=false) public String getName() {return this.name;}
	public void setName(String name) {this.name = name;}
	private SpeciesEnum speciesEnum;
	@Enumerated public SpeciesEnum getSpeciesEnum() {return speciesEnum;}
	public void setSpeciesEnum(SpeciesEnum speciesEnum) {this.speciesEnum = speciesEnum;}
	private PlanetEnum homePlanet;
	@Enumerated public PlanetEnum getHomePlanet() {return homePlanet;}
	public void setHomePlanet(PlanetEnum homePlanet) {this.homePlanet = homePlanet;}
	private AffiliationEnum affiliationEnum;
	@Enumerated public AffiliationEnum getAffiliationEnum() {return affiliationEnum;}
	public void setAffiliationEnum(AffiliationEnum affiliationEnum) {this.affiliationEnum = affiliationEnum;}
	private RankEnum rank;
	@Enumerated @NotNull public RankEnum getRank() {return rank;}
	public void setRank(RankEnum rank) {this.rank = rank;}
	private Starship starship;
	@ManyToOne public Starship getStarship() {return starship;}
	protected void setStarship(Starship starship) {this.starship = starship;}
	public Officer() {
	super();
	}
	public Officer(String name, RankEnum rank) {
	setName(name);
	setRank(rank);
	}
	@Override
	public int hashCode() {
	final int prime = 31;
	int result = 1;
	result = prime * result + ((name == null) ? 0 : name.hashCode());
	return result;
	}
	@Override
	public boolean equals(Object obj) {
	if (this == obj)
	return true;
	if (obj == null)
	return false;
	if (getClass() != obj.getClass())
	return false;
	Officer other = (Officer) obj;
	if (name == null) {
	if (other.name != null)
	return false;
	} else if (!name.equals(other.name))
	return false;
	return true;
	}
	}

view raw Officer.java hosted with ❤ by GitHub

In previous classes, we should pay attention in three important points:

• we are annotating at property level instead of field level.
• @OneToMany and @ManyToOne uses default options (apart from cascade definition)
• officers getter on Starship class returns an immutable list. 

To test model configuration, we are going to create a test which creates and persists one Starship and seven Officers, and in different Transaction and EntityManager finds created Starship.

Now that we have created this test, we can run it and we are going to observe Hibernate console output.

	Hibernate: insert into Starship (affiliationEnum, launched, height, length, power, width, registry, starshipClassEnum, id) values (?, ?, ?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Before Find Starship By Id
	Hibernate: select starship0_.id as id1_0_, starship0_.affiliationEnum as affiliat2_1_0_, starship0_.launched as launched1_0_, starship0_.height as height1_0_, starship0_.length as length1_0_, starship0_.power as power1_0_, starship0_.width as width1_0_, starship0_.registry as registry1_0_, starship0_.starshipClassEnum as starship9_1_0_ from Starship starship0_ where starship0_.id=?
	After Find Starship By Id and Before Commit
	Hibernate: select officers0_.Starship_id as Starship1_1_2_, officers0_.officers_id as officers2_2_, officer1_.id as id0_0_, officer1_.affiliationEnum as affiliat2_0_0_, officer1_.homePlanet as homePlanet0_0_, officer1_.name as name0_0_, officer1_.rank as rank0_0_, officer1_.speciesEnum as speciesE6_0_0_, officer1_.starship_id as starship7_0_0_, starship2_.id as id1_1_, starship2_.affiliationEnum as affiliat2_1_1_, starship2_.launched as launched1_1_, starship2_.height as height1_1_, starship2_.length as length1_1_, starship2_.power as power1_1_, starship2_.width as width1_1_, starship2_.registry as registry1_1_, starship2_.starshipClassEnum as starship9_1_1_ from Starship_Officer officers0_ inner join Officer officer1_ on officers0_.officers_id=officer1_.id left outer join Starship starship2_ on officer1_.starship_id=starship2_.id where officers0_.Starship_id=?
	Hibernate: delete from Starship_Officer where Starship_id=?
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	Hibernate: insert into Starship_Officer (Starship_id, officers_id) values (?, ?)
	After commit

view raw hibernate.sql hosted with ❤ by GitHub

See the number of queries executed during first commit (persisting objects) and during commit of second transaction (finding a Starship). In total and ignoring sequence generator, we can count 22 inserts, 2 selects and 1 delete, not bad when we are only creating 8 objects and 1 find by primary key.

At this point let's examine why these SQL queries are executed:

First eight inserts are unavoidable; they are required by inserting data into database.

Next seven inserts are required because we have annotated getOfficers property without mappedBy attribute. If we look closely at Hibernate documentation, it points us that “Without describing any physical mapping, a unidirectional one to many with join table is used.”

Next group of queries are even stranger, the first select statement is to find Starship by id, but what are these deletes and inserts of data that we have already created?

During commit Hibernate validates whether collection properties are dirty by comparing object references. When a collection is marked as dirty, Hibernate needs to re-create whole collection, even containing the same objects. In our case when we are getting officers we are returning a different collection instance, concretely an unmodifiable list, so Hibernate considers officers collection as dirty.

Because a join table is used, Starship_Officer table should be re-created, deleting previous inserted tuples and inserting the new ones (although they have the same values).

Let's try to fix this problem. We start by mapping a bidirectional one-to-many association, with many-to-one side as owning side.

	private List<Officer> officers = new ArrayList<Officer>();
	@OneToMany(mappedBy="starship", cascade={CascadeType.ALL}) public List<Officer> getOfficers() {return Collections.unmodifiableList(officers);}
	protected void setOfficers(List<Officer> officers) {this.officers = officers;}
	public void addOfficer(Officer officer) {this.officers.add(officer);}

view raw Starship.java hosted with ❤ by GitHub

And now we rerun the same test again and we inspect the output again.

	Hibernate: insert into Starship (affiliationEnum, launched, height, length, power, width, registry, starshipClassEnum, id) values (?, ?, ?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Before Find Starship By Id
	Hibernate: select starship0_.id as id1_0_, starship0_.affiliationEnum as affiliat2_1_0_, starship0_.launched as launched1_0_, starship0_.height as height1_0_, starship0_.length as length1_0_, starship0_.power as power1_0_, starship0_.width as width1_0_, starship0_.registry as registry1_0_, starship0_.starshipClassEnum as starship9_1_0_ from Starship starship0_ where starship0_.id=?
	After Find Starship By Id and Before Commit
	Hibernate: select officers0_.starship_id as starship7_1_1_, officers0_.id as id1_, officers0_.id as id0_0_, officers0_.affiliationEnum as affiliat2_0_0_, officers0_.homePlanet as homePlanet0_0_, officers0_.name as name0_0_, officers0_.rank as rank0_0_, officers0_.speciesEnum as speciesE6_0_0_, officers0_.starship_id as starship7_0_0_ from Officer officers0_ where officers0_.starship_id=?
	After commit

view raw hibernate.sql hosted with ❤ by GitHub

Although we have reduced the number of SQL statements, from 25 to 10, we still have an unnecessary query, the ones just in commit section of second transaction. Why if officers are lazy by default (JPA specification), and we are not getting officers in transaction, Hibernate executes a select on Officers table?  By the same reason as previously configuration, returned collection has different Java identifier, so Hibernate marks it as newly instantiated collection, but now obviously join table operations are no longer required. We have reduced the number of queries but we still have a performance problem. It is likely that we'll need some other solution, and the solution is not the most obvious one, we are not going to return collection objects returned by Hibernate, we might expand on this later, but we are going to change annotations location.

What we are going to do is to change mapping location from property approach to use field mapping. Simply we are going to move all annotations to class attributes rather than on getters.

	@Entity
	public class Starship {
	@Id @GeneratedValue(strategy=GenerationType.SEQUENCE)
	private Long id;
	public Long getId() {return id;}
	protected void setId(Long id) {this.id = id;}
	@Temporal(TemporalType.DATE) private Date launched;
	public Date getLaunched() {return launched;}
	public void setLaunched(Date launched) {this.launched = launched;}
	...
	@OneToMany(mappedBy="starship", cascade={CascadeType.ALL})
	private List<Officer> officers = new ArrayList<Officer>();
	public List<Officer> getOfficers() {return Collections.unmodifiableList(officers);}
	protected void setOfficers(List<Officer> officers) {this.officers = officers;}
	public void addOfficer(Officer officer) {
	officer.setStarship(this);
	this.officers.add(officer);
	}
	public Starship() {
	super();
	}
	public Starship(String registry) {
	setRegistry(registry);
	}
	@Override
	public int hashCode() {
	final int prime = 31;
	int result = 1;
	result = prime * result
	+ ((registry == null) ? 0 : registry.hashCode());
	return result;
	}
	@Override
	public boolean equals(Object obj) {
	if (this == obj)
	return true;
	if (obj == null)
	return false;
	if (getClass() != obj.getClass())
	return false;
	Starship other = (Starship) obj;
	if (registry == null) {
	if (other.registry != null)
	return false;
	} else if (!registry.equals(other.registry))
	return false;
	return true;
	}
	}

view raw Starship.java hosted with ❤ by GitHub

	@Entity
	public class Officer {
	@Id @GeneratedValue(strategy=GenerationType.SEQUENCE) private Long id;
	public Long getId() {return id;}
	protected void setId(Long id) {this.id = id;}
	@Column(unique=true, nullable=false) private String name;
	public String getName() {return this.name;}
	public void setName(String name) {this.name = name;}
	@Enumerated private SpeciesEnum speciesEnum;
	public SpeciesEnum getSpeciesEnum() {return speciesEnum;}
	public void setSpeciesEnum(SpeciesEnum speciesEnum) {this.speciesEnum = speciesEnum;}
	...
	@ManyToOne private Starship starship;
	public Starship getStarship() {return starship;}
	protected void setStarship(Starship starship) {this.starship = starship;}
	public Officer() {
	super();
	}
	public Officer(String name, RankEnum rank) {
	setName(name);
	setRank(rank);
	}
	@Override
	public int hashCode() {
	final int prime = 31;
	int result = 1;
	result = prime * result + ((name == null) ? 0 : name.hashCode());
	return result;
	}
	@Override
	public boolean equals(Object obj) {
	if (this == obj)
	return true;
	if (obj == null)
	return false;
	if (getClass() != obj.getClass())
	return false;
	Officer other = (Officer) obj;
	if (name == null) {
	if (other.name != null)
	return false;
	} else if (!name.equals(other.name))
	return false;
	return true;
	}
	}

view raw Officer.java hosted with ❤ by GitHub

And finally we are going to run the test again, and see what's happen:

	Hibernate: insert into Starship (affiliationEnum, launched, height, length, power, width, registry, starshipClassEnum, id) values (?, ?, ?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Hibernate: insert into Officer (affiliationEnum, homePlanet, name, rank, speciesEnum, starship_id, id) values (?, ?, ?, ?, ?, ?, ?)
	Before Find
	Hibernate: select starship0_.id as id1_0_, starship0_.affiliationEnum as affiliat2_1_0_, starship0_.launched as launched1_0_, starship0_.height as height1_0_, starship0_.length as length1_0_, starship0_.power as power1_0_, starship0_.width as width1_0_, starship0_.registry as registry1_0_, starship0_.starshipClassEnum as starship9_1_0_ from Starship starship0_ where starship0_.id=?
	After Find Before Commit
	After commit

view raw hibernate.sql hosted with ❤ by GitHub

Why using property mapping Hibernate runs queries during commit and using field mapping are not executed? When a Transaction is committed, Hibernate execute a flush to  synchronize the underlying persistent store with persistable state held in memory. When property mapping is used, Hibernate calls getter/setter methods to synchronize data, and in case of getOfficers method, it returns a dirty collection (because of unmodifiableList call). On the other side when we are using field mapping, Hibernate gets directly the field, so collection is not considered dirty and no re-creation is required.

But we have not finished yet, I suppose you are wondering why we have not removed Collections.unmodifiableList from getter, returning Hibernate collection? Yes I agree with you that we finished quickly, and change would look like @OneToMany(cascade={CascadeType.ALL}) public List<Officer> getOfficers() {officers;} but returning original collection ends up with an encapsulation problem, in fact we are broken encapsulation!. We could add to mutable list anything we like; we could apply uncontrolled changes to the internal state of an object.

Using an  unmodifiableList is an approach to use to avoid breaking encapsulation, but of course we could have used different accessors for public access and hibernate access, and not calling  Collections.unmodifiableList method.

Considering what we have seen today, I suggest you to use always field annotations instead of property mapping, we are going to save from a plenty of surprises.

Hope you have found this post useful.

Screencast of example shown here:



Download code
Music: http://www.youtube.com/watch?v=H14VIsnr6aA

Keep 'em laughing as you go, Just remember that the last laugh is on you, And always look on the bright side of life..., Always look on the right side of life... (Always Look on the Bright Side of Life - Mony Python)

Integration tests are kind of tests which individual modules are combined and tested as a whole. Moreover integration tests might use system dependent values, accessing external systems like file system, database, web services, ..., and testing multiple aspects of one test case. We can say it is a high-level test.

This differs from unit test where only a single component is tested. Unit tests runs in isolation, mocking-out external components or using in-memory database in case of DAO layers. A unit test might be:

• Repeatable.
• Consistent.
• In Memory.
• Fast.
• Self-validating.
• Testing single concept

The problem when we are writing tests, is how to test rare (or untypical) conditions like "No disk space" in case of accessing file system, or "Connection lost" when executing a database query.

In unit testing this is not a problem you can mock up that component (database connection or filesystem access), generating required output like throwing IOException.

The problem becomes "harder" with integration tests. It would be strange to mock a component, when what you really want to do is validate the real system. So arrived at this point I see two possibilities:

• Creating a partial mock.
• Using fault injection.

In this post I am going to show you how to use fault injection approach to test unusual erroneous situations. 

Fault injection is a technique which involves changing application code under test at specific locations. This modifications will introduce faults on error handling code paths which otherwise would rarely be followed.

I am going to talk about how to use fault injection using Byteman in a JUnit test, and run it with Maven.

Let's start coding. Imagine you need to write a backup module, which shall save a string into a local file, but if hard disk is full (IOException is thrown), content shall be sent to remote server.

First we are going to code a class that writes content into file.

	public class FileUtils {
	public void createFileWithContent(File filename, String content) throws IOException {
	FileWriter fileWriter = new FileWriter(filename);
	BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
	bufferedWriter.write(content);
	bufferedWriter.flush();
	bufferedWriter.close();
	}
	}

view raw FileUtils.java hosted with ❤ by GitHub

Next class, would be the one that sends data through socket but will not be shown, because it is not necessary for this example.

And finally the backup service responsible of managing described behavior.

	public class BackupManagerService implements BackupManager {
	static final String BACKUP_NAME_FILE = "backup";
	private File backupDirectory;
	private String hostname;
	private FileUtils fileUtils;
	private SocketUtils socketUtils;
	public BackupManagerService(File backupDirectory, String hostname) {
	this.backupDirectory = backupDirectory;
	this.hostname = hostname;
	}
	@Override
	public void backupData(String content) {
	try {
	this.fileUtils.createFileWithContent(new File(backupDirectory, BACKUP_NAME_FILE), content);
	} catch (IOException e) {
	this.socketUtils.sendData(hostname, content);
	}
	}
	public void setFileUtils(FileUtils fileUtils) {
	this.fileUtils = fileUtils;
	}
	public void setSocketUtils(SocketUtils socketUtils) {
	this.socketUtils = socketUtils;
	}
	}

view raw BackupManagerService.java hosted with ❤ by GitHub

And now testing time. First of all a brief introduction to Byteman.

Byteman is a tool which allows you to insert/modify code into an application at runtime. These modifications can be used to inject code on your compiled application causing unusual or unexpected operations (aka Fault Injection).

Byteman uses a clear, simple scripting language, based on a formalism called Event Condition Action (ECA) rules to specify where, when and how the original Java code should be transformed.

An example of ECA script is:

	RULE throw on Nth empty get
	CLASS org.my.BoundedBuffer
	METHOD get()
	AT INVOKE Object.wait()
	BIND buffer = $this
	IF countDown(buffer)
	DO throw new org.my.ClosedException(buffer)
	ENDRULE

view raw rule.btm hosted with ❤ by GitHub

But Byteman also supports annotations. And in my opinion, annotations are a better approach than script file, because only watching your test case you can understand what you are exactly testing. If not you should switch context from unit class to script file to understand what are you testing.

So let's create an integration test that that validates that when IOException is thrown while writing content into disk, data is sent to a server.

	@RunWith(BMUnitRunner.class)
	public class CreateFileWithContentTest {
	@Rule
	public TemporaryFolder folder = new TemporaryFolder();
	@Mock
	private SocketUtils socketUtils;
	@Before
	public void setup() {
	MockitoAnnotations.initMocks(this);
	}
	@Test
	public void aFileWithContentShouldBeCreated() throws IOException {
	BackupManager backupManager = createBackupManager();
	backupManager.backupData("Hello World");
	assertThat(readFully(new File(folder.getRoot(), BackupManagerService.BACKUP_NAME_FILE)), is("Hello World"));
	}
	@Test
	@BMRule(
	name="throw IOException writting content",
	targetClass = "com.lordofthejars.byteman.util.FileUtils",
	targetMethod = "createFileWithContent",
	targetLocation = "CALL BufferedWriter.write(String)",
	action = "throw new java.io.IOException()"
	)
	public void dataShouldBeSentToServerInCaseOfIOException() throws IOException {
	BackupManager backupManager = createBackupManager();
	backupManager.backupData("Hello World");
	verify(this.socketUtils).sendData("myhost", "Hello World");
	}
	private BackupManagerService createBackupManager() {
	FileUtils fileUtils = new FileUtils();
	BackupManagerService backupManager = new BackupManagerService(folder.getRoot(), "myhost");
	backupManager.setFileUtils(fileUtils);
	//Socket util class is not implemented and will be mocked but of course this would not be the case in real world.
	backupManager.setSocketUtils(this.socketUtils);
	return backupManager;
	}
	private static String readFully(File filename) throws IOException {
	FileReader fileReader = new FileReader(filename);
	BufferedReader bufferedReader = new BufferedReader(fileReader);
	String content = bufferedReader.readLine();
	return content;
	}
	}

view raw CreateFileWithContentTest.java hosted with ❤ by GitHub

See that BMUnitRunner (a special jUnit runner that comes with Byteman) is required.

First test called aFileWithContentShouldBeCreated is a standard test that writes Hello world into backup file.

But the second one dataShouldBeSentToServerInCaseOfIOException, has BMRule annotation which will contain when, where and what code should be injected. First parameter is the name of the rule, in this case a description of what we are going to do (throwing an IOException). Next attributes, targetClass and targetMethod configure when injected code should be added. In this case when FileUtils.createFileWithContent method is called. Next attribute targetLocation is location where code is inserted, and in our case is where createFileWithContent method calls write method of BufferedWriter. And finally what to do that obviously in this test is throwing an IOException.

So now you can go to your IDE and run them, and all tests should pass, but if you run through Maven using Surefire plugin, test will not work. To use Byteman with Maven, Surefire plugin should be configured in a specific way.

	<properties>
	<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
	<tools.jar>${java.home}/../lib/tools.jar</tools.jar>
	</properties>
	<plugin>
	<groupId>org.apache.maven.plugins</groupId>
	<artifactId>maven-surefire-plugin</artifactId>
	<version>2.4.3</version>
	<configuration>
	<!-- ensure we don't inherit a byteman jar form any env settings -->
	<environmentVariables>
	<BYTEMAN_HOME></BYTEMAN_HOME>
	</environmentVariables>
	<systemProperties>
	<property>
	<name>org.jboss.byteman.home</name>
	<value></value>
	</property>
	</systemProperties>
	<useManifestOnlyJar>false</useManifestOnlyJar>
	<forkMode>once</forkMode>
	<parallel>false</parallel>
	</configuration>
	</plugin>
	<dependency>
	<groupId>org.jboss.byteman</groupId>
	<artifactId>byteman</artifactId>
	<scope>test</scope>
	<version>2.0.0</version>
	</dependency>
	<dependency>
	<groupId>org.jboss.byteman</groupId>
	<artifactId>byteman-submit</artifactId>
	<scope>test</scope>
	<version>2.0.0</version>
	</dependency>
	<dependency>
	<groupId>org.jboss.byteman</groupId>
	<artifactId>byteman-install</artifactId>
	<scope>test</scope>
	<version>2.0.0</version>
	</dependency>
	<dependency>
	<groupId>org.jboss.byteman</groupId>
	<artifactId>byteman-bmunit</artifactId>
	<scope>test</scope>
	<version>2.0.0</version>
	</dependency>
	<dependency>
	<groupId>com.sun</groupId>
	<artifactId>tools</artifactId>
	<version>1.6</version>
	<scope>system</scope>
	<systemPath>${tools.jar}</systemPath>
	</dependency>

view raw pom.xml hosted with ❤ by GitHub

First important thing is adding tools jar as dependency. This jar provides classes needed in order to dynamically install the Byteman agent.

In Surefire plugin configuration is important to set useManifestOnlyJar to false to ensure that the Byteman jar appears in the classpath of the test JVM.  Also see that we are defining empty environment variables (BYTEMAN_HOME and org.jboss.byteman.home). This is because when it loads the agent the BMUnit package will use environment variable BYTEMAN_HOME or System property org.jboss.byteman.home to locate byteman.jar but only if it is a non-empty string. Otherwise it scans the classpath to locate the jar. Because we want to ensure that jar added on dependency section is used, we are overriding any other configuration present on system.

And now you can run mvn clean test and two tests are successful too.

See that Byteman opens a new world into how we are writing our integration tests, now we can test in an easy way unusual exceptions like Communications Error, Input/Output Exceptions or Out Of Memory Error. Moreover because we are not mocking FileUtils, we are executing real code; for example in our second test, we are running a few lines of FileUtils object until write method is reached. If we had mocked-up FileUtils class, these lines would not be executed. Thanks of using fault injection our code coverage is improved.

Byteman is more than what I have shown you, it also has built-ins designed for testing in multithreaded environments, parameter binding, and an amount of location specifiers, to cite a few things.

I wish you have found this post useful and help you testing rare conditions of your classes.

Download Code

Music: http://www.youtube.com/watch?v=WlBiLNN1NhQ