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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Loglama işleminin "spy appender" yoluyla test edilmesi

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Loglama işlemi yapan bir sınıf, Logger nesnesini şu şekilde elde ediyor olsun:

Logger log = Logger.getLogger(FinalExceptionHandlerImpl.class.getName());

Burada loglama işleminin yapılıp yapılmadığını test edebilmek için, Logger sınıfına
bir "casus" appender enjekte ediyoruz. Sonuçta test sınıfımız aşağıdaki şekilde oluyor:

@RunWith(MockitoJUnitRunner.class)
public class FinalExceptionHandlerImplTest {
private SpyAppender spyAppender = new SpyAppender();

FinalExceptionHandlerImpl handler;

@Mock
private MessagesUIConnector messagesUIConnector;

@Before
public void setUp() throws Exception {
handler = new FinalExceptionHandlerImpl();
Logger.getLogger(FinalExceptionHandlerImpl.class.getName())
.addAppender(spyAppender);
}

@Test
public void shouldLogErrorForException() throws Exception {
Exception exception = new IndexOutOfBoundsException("message");

handler.handleException(exception, messagesUIConnector);

List messagesLogged = spyAppender.getMessagesLogged();

assertThat(messagesLogged.size(), equalTo(1));
assertThat(messagesLogged.get(0).toString(), equalTo("java.lang.IndexOutOfBoundsException: message"));
}

Spy Appender sınıfı aşağıdaki şekilde yazılabilir:

class SpyAppender extends AppenderSkeleton {
private List messages = Lists.newArrayList();

protected void append(LoggingEvent loggingEvent) {
messages.add(loggingEvent.getRenderedMessage());
}

public List getMessagesLogged() {
return messages;
}

public void close() {
}

public boolean requiresLayout() {
return false;
}
}

Böylece loglama işlemi ile ilgili birçok ayrıntıyı, SpyAppender sınıfını geliştirerek test etmemiz mümkün görünüyor.

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