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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A Code Analyzer for a Subset of Java Language (J-) Using ANTLR - PART 2

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As we know, semantic analysis of a compiler includes checking the types of expressions.
To do that, we have to maintain a Symbol Table which maps variable names to their types and locations. There maybe many variables with the same name which are in different scopes. This is legal in most languages and we have to handle this situation.

We have to know when following scopes are entered in:
- class scope
- method scope
- no inner block scopes (my language does not support this currently)

We will do type-checking in two phases. First we'll populate the Symbol Table and it will contain the symbols with their scopes. Then, we'll check the types of symbols that are subject to addition operation by querying the symbol table.

ANTLR allows us to inject code in different places of the generated parser. We need to know when a class/method scope is entered and the end of any scope is reached. Below is some grammar code from the project:


classDecl
@init {visitor.beforeClass(); System.out.println("class classScope started");}
@after {visitor.afterClass(); System.out.println("class classScope ended");}
: ^(CLASS ID varDecl* methodDecl*) {visitor.onClassName($ID.text); System.out.println("class is: " + $ID.text);}
;

methodDecl
@init {visitor.beforeMethod(); System.out.println("method classScope started");}
@after {visitor.afterMethod(); System.out.println("method classScope ended");}
: ^(METHOD type ID formalParameter* block) {visitor.onMethodName($ID.text);}
;

We inject a visitor to the parser because we don't want to clutter the tree grammar code with type-checking details.

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