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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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ANTLR ile Lexer Generation

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Java platformunda bir lexer oluşturmak istediğimizde elimizdeki seçenekler JavaCC, SableCC ve ANTLR olarak karşımıza çıkıyor. ANTLR ile elimizdeki dil için bir lexer oluşturmaya çalışalım. Lexer için kullanacağımız dilin token kuralları: Identifier: Harf ile başlar. Harf, rakam ve alt çizgilerden oluşan stringdir. Büyük haft – küçük harf farklı olarak değerlendirilir. Integer constant: Rakamlardan oluşan string. Binary operator: Aşağıdakilerden biri: + - * / && Yorum: İki türlü yorum olabilir: Blok tipi yorum: /* */ Satır tipi yorum: // ANTLR kurulumu için yapılması gerekenler: Antlrworks'ü indir ve üzerine tıkla. En sevdiğin kafeinli içecek de dahil olmak üzere bilgilerini göndermeyi kabul et. ;) Yeni gramer dökümanı mı oluşturmak istiyorsun? Evet Grammar name: MyLexer Type: Lexer Lexical items: Identifier, Integer, Comments (single line + multiline), Whitespace Oluşturacağımız lexer için belirlenen kuralları ana pencerede görüyoruz: ID : ('a'..'z...
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"REGULAR EXPRESSIONS" KONUSUNU TAM OLARAK VE KOLAYCA ANLAMAK

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Herhangi bir dilde yazılmış bilgisayar programının derlenebilmesi veya yorumlanabilmesi için öncelikle "lexical analysis" denilen aşamadan geçmesi gerekiyor. Bu aşamada program kelimelere veya "token" denilen yapıtaşlarına ayrılıyor. Programlama dilinin sentaksı, tokenlerin türleri hakkında bilgi verir. Örneğin birçok dilde yer alan bazı token türleri: ID Örnek: price calculateRate lastItem NUM Örnek: 73 0 REAL Örnek: 12.5 0.23 COMMA , IF if Bir lexical analyser (lexer) yazabilmek için herhangi bir programlama dili kullanılabilir. Fakat özellikle "regular expressions" dili bu iş için kullanıma uygun. Regular Expressions Dil (language), bir string kümesi olarak tanımlansın. Örneğin Java dili, legal java programı oluşturabilecek tüm string lerin kümesidir. Bu durumda Java dili sonsuz elemana sahip bir küme. Peki bir dili (belki sonsuz elemana sahip), nasıl sonlu ifadelerle (açıklamalarla) tanımlayabiliriz? İşte burad...
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nijhof2axon

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Axon framework kullanıcılarına katkıda bulunmak amacıyla paylaştığım proje ile ilgili yorum ve sorular gelmeye başladı. http://code.google.com/p/nijhof2axon/ >>You share the Data between the Domain and the Query Model? When a command is dispatched, a CommandHandler uses the aggregate root to handle the command. Aggregate root fires DomainEvents. => Query layer related code listens to domain events and updates the query model. (Example: ClientTableUpdater) Domain entities are first class objects but Query Model classes are just DTOs with getters and setters. We have Client as entity and ClientDetailsEntry as DTO. >>The (generated?) table domainevententry looks like you can not replay events? missing the event data? To observe how events are replayed by the framework, you can follow these steps: Click on sample client change name stop server start server put a breakpoint to EventSourcingRepository.doLoad() method put a breakpoint to Client.handleClientNameChangedEvent() me...
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The WeakReference class, monitoring memory leak and garbage collection in a Java application

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 Below is a Stack implementation that uses an internal resizeable array structure.  public class MyStack< T > implements Stack< T > { private static final int CAPACITY = 100 ; private Object[] array ; private int pos = 0 ; public MyStack () { this . array = new Object[ CAPACITY ] ; } @Override public void push ( T item) { if ( pos >= array . length / 2 ) { Object[] newArray = new Object[ pos * 2 ] ; System. arraycopy ( array , 0 , newArray , 0 , array . length ) ; array = newArray ; } array [ pos ++] = item ; } @Override public T pop () { if (isEmpty()) { throw new RuntimeException( "empty stack" ) ; } @SuppressWarnings ( "unchecked" ) T item = ( T ) array [ pos - 1 ] ; pos -= 1 ; return item ; } @Override @SuppressWarnings ( "unchecked" ) public T peek...
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Simplescalar Simulator - Part 2: sim-outorder.c

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sim-outorder implements a very detailed out-of-order issue superscalar processor with a two-level memory system and speculative execution support. This simulator is a performance simulator, tracking the latency of all pipeline operations. Let's have a look at SimpleScalar simulator's code related to iTLB. (Instruction Translation Lookaside Buffer) First, there are simulator options related to iTLB: /* register simulator-specific options */ void sim_reg_options(struct opt_odb_t *odb) { /* TLB options */ opt_reg_string(odb, "-tlb:itlb", "instruction TLB config, i.e., { |none}", &itlb_opt, "itlb:16:4096:4:l", /* print */TRUE, NULL); opt_reg_int(odb, "-tlb:lat", "inst/data TLB miss latency (in cycles)", &tlb_miss_lat, /* default */30, /* print */TRUE, /* format */NULL); } /* check simulator-specific option values */ void  sim_check_options(... ) { /* sim-outorder.c  line: 1098*/   /* u...
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Notes on Java Performance

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Performance monitoring, performance profiling and performance tuning are different activities. For example, monitoring is not an intrusive action but profiling is. Because it may be changing the responsiveness of the running application. User CPU utilization vs Kernel CPU utilization: The ideal situation is when Kernel CPU utilization is 0%. So ideally, all the CPU cycles must be spent on our application code. JVM just in time compiler performs dynamic optimizations. So it makes decisions while the program is running and generates native code with better performance. Java 1.0 was completely interpreted but beginning with Java 1.1.8, there was a just-in-time compiler. It was 8 times faster. After that, paralellizing the garbage collector was a huge improvement. There are hundreds of Java tuning flags. https://github.com/ScottOaks/JavaPerformanceTuning Boolean flags use this syntax: -XX:+FlagName enables the flag, and -XX:-FlagName disables the flag. 32 BIT VS 64 BIT Fr...
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