Observability of the Java Virtual Machine

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The JVM is one of the most observable runtimes. It provides us lots of tools for troubleshooting a JVM application in production. 1. Thread observability Threads are how the JVM actually does work. When something is wrong in production, the symptom is almost always a thread: stopped, blocked, leaking etc. Thread dumps work on any JVM with no  instrumentation, no agents, no restarts. <Example project link with /threaddump endpoint>         // (1) Deadlock — two threads grab the same pair of locks in opposite order.         new Thread(() -> grab(LOCK_A, LOCK_B), "deadlock-A-then-B").start();         new Thread(() -> grab(LOCK_B, LOCK_A), "deadlock-B-then-A").start(); http://localhost:8080/actuator/threaddump To list the JVMS, we can use the command below. PS C:\observe-jvm> jps -lv 25296 jdk.jcmd/sun.tools.jps.Jps -Dapplication.home=C:\Program Files\Microsoft\jdk-21.0.3.9-hotspot -Xms8m -Djdk.module.main=...
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Mockist testler ve custom matcher yazımı

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Çalıştığım projede Mockito testlerini yoğun olarak kullanıyoruz ve birçok durumda custom matcher yazmamız sözkonusu oluyor.

Aşağıdaki mock test örneğine bakalım:

@Test
public void saveSkillAssignmentApprovalShouldCreateNew() throws ServiceException {
Skill skill = new SkillBuilder().buildSample();
SkillAssignmentDao dao = mock(SkillAssingmentDao.class);
//...


// manager kendi icerisinde dao.save(..) metodunu cagiriyor.
SkillAssignment result = skillAssignmentManager.saveSkillAssignmentApproval(approver, skill, skillLevel);

// burada assertion icin result'i kullanamayiz cunku mock dao tarafindan set //ediliyor.
assertThat(result.getSkill(),equalTo(skill)); // ??
}

Burada test ettiğimiz skillAssignmentManager nesnesinin geri gönüş değeri yoluyla sınama yapamayacağımız açık. O nedenle mock olarak kullandığımız dao metoduna geçilen parametreyi sınamamız gerekiyor:

@Test
public void saveSkillAssignmentApprovalShouldCreateNew() throws ServiceException {
SkillAssignmentDao skillAssignmentDao = mock(SkillAssingmentDao.class);
//...


// manager kendi icerisinde dao.save(..) metodunu cagiriyor.
skillAssignmentManager.saveSkillAssignmentApproval(approver, skill, skillLevel);

IsExpectedAssignment isExpectedAssignment = new IsExpectedAssignment( skill);

verify(skillAssignmentDao).save(argThat(isExpectedAssignment));

}

class IsExpectedAssignment extends ArgumentMatcher {
private Skill skill;

public IsExpectedAssignment(Skill skill) {
this.skill = skill;
}

public boolean matches(Object skillAssignment) {
SkillAssignment actual = (SkillAssignment) skillAssignment;

if (!actual.getSkill.equals(skill))
return false;

//...
}

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