jvm - Kenyth Zeng on Diigo

08 Dec 09

动态地为一个已经启动的java附加一个agent上去.

import com.sun.btrace.annotations.*;
import static com.sun.btrace.BTraceUtils.*;

@BTrace
public class HelloWorld {
    @OnMethod(
        clazz="java.lang.Thread",
        method="start"
    )

    public static void func() {
        println("about to start a thread!");
    }
}

@OnMethod告诉Btrace解析引擎需要代理的类和方法。
这个例子的作用是当java.lang.Thread类的任意一个对象调用 start 方法后，会调用 func 方法。

1 more annotations...
- inst.retransformClasses(java.lang.Thread.class);

06 Nov 09

interface injection in the VM - John Rose @ Sun

technology design preview

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interface jvm feature blog injection class

when I hear customers ask for an API to edit classes in the JVM, I always reach for an alternative, a prescription subtitute for the illegal substance.
Not only do you have to load the new code, you have to undo the relevant effects of the old code, and there is always the temptation to “diff” the old and new versions, so as to avoid undoing and redoing everything. But diff-patching something that complex leads you down a long path of painful bugs.
2 more annotations...
- Interface injection (in the JVM) is the ability to modify old classes just enough for them to implement new interfaces which they have not encountered before.
- The original form of JSR 292 includes a vague promise of considering features for class extension.

Class Loading in Java: Time to Add Dynamicity? | Javalobby

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JRebel hot deploy java discussion .net jvm classloader class article

But not everything can be reloaded by JavaRebel: No reloads with class hierarchy change are possible.
no schema changes are allowed.
8 more annotations...
- Our «irritating little» friend Ruby has this idea of open classes: you may change a class at runtime by adding new methods. This proves that it's possible to do ad-hoc changes.
- Dynamic languages pay a huge performance hit for the ability to update classes dynamically.
- http://classworlds.codehaus.org/
- AFAIK .NET does not reload code, it does full redeploy juts like Java, just application start usually will take less.
- In many cases if you have problems with proxies adding "-Drebel.forbid_bypass=true" will help.
- So if we could only have a jvm-server that could integrate all types of similar jar in the memory.
- They're all compiled with offsets, not labels. When you say "getTitle", the JVM knows the offset within the object structure to the "title" instance variable (if you had, say, 3 instance variables, title being the 3rd slot, all class references (vs, say, char or int),
- Most dynamic languages written on top of the JVM, for example, don't use Java Classes at all for their instances, because they're so rigid.

How JavaRebel Saves Development Time | Javalobby

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JRebel java hot deploy classloader jvm debug dev article how-something-work starred

When debugging through my code in Eclipse, it’s nice to make a change and see that reflected in the running instance straight away with HotSwap.
I’ve often wondered why this is the case with Java when dynamic languages and .NET can handle this quickly.
8 more annotations...
- Finally, OSGi is another approach to take - by splitting your application into smaller modules, it can make the redeployment seem faster.
- With JavaRebel, it’s conceivable that I can start up my debug session and nicely interweave my code changes into the running instance, without anything unexpected happening.
- Why doesn’t this exist already in Java? And how has .NET managed to have this from day one?
- The tricky part in reloading object-oriented code is preserving the object state intact.
- However web development frameworks in .NET are designed around stateless and/or isolated classes so turnaround is usually instantaneous. However I’d assume that writing a complicated business app would run into same problem as Java at some point.
- The need to support class code, structure and layout changes would mean significant compromises in the JVM Just-In-Time compiler. Specifically you’d have to do dependency analysis between classes and make the method lookup and calls more dynamic than they currently are. This would decrease performance, or at least further complicate the design of the JVM. These compromises are already present in dynamic languages VMs, which makes it easy to implement such features there.
- We extend their APIs with an ability to inject and override classes, resources and web resources in the running web applications. We then interpret the rebel.xml file provided by the user to override the deployed classes and resources with the ones found from the project workspace.
- To reload the actual classes we build a kind of our own Just-In-Time compiler, which intercepts loading of every class and rewrites them in a way that makes reloading possible.

JRebel FAQ | ZeroTurnaround.com

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java hot deploy deploy classloader memory leak jvm debug dev FAQ

without having to restart the
container or redeploy the application.
It does not create any new class loaders,
instead, it
extends the existing ones with the ability to manage reloaded classes.
3 more annotations...
- The .class
  file timestamp is then
  monitored for changes in the loaded class and updates are propagated
  through the extended class loader, to your application.
- Importantly, when loading updates to a class, JRebel preserves all
  of
  the existing instances of that class. This allows the application to
  just continue working, but also means that when adding a new instance
  field it will not be initialized in the existing instances, since the
  constructor will not be rerun.
- The Instrumentation API was introduced in Java
  5 and included a limited ability to redefine Java classes on-the-fly.
  Unfortunately it is limited to only changing method bodies (as is
  HotSwap)

JRebel Feature Comparison Matrix | ZeroTurnaround.com

Hot Redeploy JVM HotSwap JRebel

www.zeroturnaround.com/...comparison - Preview

feature comparison hot deploy deploy classloader memory leak starred java jvm debug dev

02 Nov 09

The Programming Delusion: I am Visible but am I Accessible?

The difficult part is to define the module boundary so the VM can enforce access and this is still an area of ongoing discussion in the JSR 294 EG. Related to this is also the desire to
have Java compilers begin to understand module boundaries with respect to visibility and also the new module accessibility keyword.

blog.bjhargrave.com/...sible-but-am-i-accessible.html - Preview

java type jvm programming language OSGi modularization starred blog JSR 294

the distinction between visibility and accessibility in Java.
Visibility is whether one type can see another type. In the JLS, this is discussed as observability. In the JVMS, this is about class loading. Basically, this can be described as whether type T is visible to type S. At compile time, this means that the compiler can locate type T when compiling type S. At runtime, this means that the class loader for type S can load (either directly or through delegation) type T. A type can also be visible through reflection. Even if the class loader of type S cannot load type T, type S may come across type T while reflecting. For example, an object may be of type T even though type S directly refers to it via an interface type I.
4 more annotations...
- Accessibility is whether one type can access another type or a member of another type. This is discussed in the JLS and the JVMS. Most people know accessibility by the public, protected, and private keywords.
- If that class has public methods which are not part of the service interface, bundle A is able to call them.
- Adding a module accessibility keyword
- have Java compilers begin to understand module boundaries with respect to visibility and also the new module accessibility keyword.

>: Type Checking - Java, Fast !

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type java jvm blog 5.0 performance

Prior to 5, Every Subtype has cached with its SuperType(casting of which is correct).
It try to understand lot of code at compile time only. It put the entire code inline and there is no requirement of VM calls.
1 more annotations...
- divide the subtype checks into primary and secondary checks.

12 Oct 09

swarm-dpl - Project Hosting on Google Code

a framework serves as a true distributed programming language.

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distributed p2p opensource innovation starred Scala jvm scalability framework ideas

Watch a video introducing Swarm (36 minutes):

22 Sep 09

Scala Distribution | The Scala Programming Language

www.scala-lang.org/downloads - Preview

programming language jvm opensource scripting

scala-tool-support^(1,2) Tool support files for various text editors like emacs, vim or gedit

17 Sep 09

noop - Project Hosting on Google Code

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testing programming language jvm injection

2 more annotations...
- The goal is to build dependency injection and testability into the language from the beginning, rather than rely on third-party libraries as all other languages do.
- making final/const behavior the default and providing easy access to a functional style. Testability is encouraged by providing Dependency Injection at the language level and a compact constructor injection syntax.

24 Aug 09

MemoryMXBean (Java Platform SE 6)

java.sun.com/...MemoryMXBean.html - Preview

jvm memory api javadoc

reclaimed by an automatic memory management
system which is known as a garbage collector.
The method area belongs to non-heap memory. It stores per-class structures
such as a runtime constant pool, field and method data, and the code for
methods and constructors. It is created at the Java virtual machine
start-up.
2 more annotations...
- The Java virtual machine has a method area that is shared
  among all threads.
- In addition to the method area, a Java virtual machine
  implementation may require memory for internal processing or
  optimization which also belongs to non-heap memory.
  For example, the JIT compiler requires memory for storing the native
  machine code translated from the Java virtual machine code for
  high performance.

12 Aug 09

VM Spec Threads and Locks

java.sun.com/...Threads.doc.html - Preview

jvm java spec memory thread concurrent starred

Because it is impossible
for one thread to access parameters or local variables of another thread, it does
not matter whether parameters and local variables are thought of as residing in the
shared main memory or in the working memory of the thread that owns them.
Every thread has a working memory in which it keeps its own working copy of variables that it must use or assign. As the thread executes a program, it operates on these working copies. The main memory contains the master copy of every variable. There are rules about when a thread is permitted or required to transfer the contents of its working copy of a variable into the master copy or vice versa.
4 more annotations...
- the operations in main memory on behalf of any one thread are performed in the same order as the corresponding operations by that thread.
- an occurrence of V as an operand of the + operator requires that a single use operation occur on V ; an occurrence of V as the left-hand operand of the assignment operator = requires that a single assign operation occur.
- If the following rules forbid T to perform a required use as its next action, it may be necessary for T to perform a load first in order to make progress.
- treated as if it were two variables of
  32 bits each; wherever the rules require one of these operations, two such operations
  are performed, one for each 32-bit half.

18 Jun 09

Liweb 0.1.0 - Demo

ajax framework choice

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Liweb ajax framework java GWT bytecode jvm GPL opensource comparison

09 May 09

Monitoring and Management Using JMX Technology - Java SE Monitoring and Management Guide

java.sun.com/...agent.html - Preview

JMX tutorial remote java jvm monitor

To enable monitoring and management from remote systems, you must set the following
system property when you start the Java VM.
```
com.sun.management.jmxremote.port=portNum
```
Remote monitoring and management requires security, to ensure that unauthorized persons cannot control
or monitor your application. Password authentication over the Secure Sockets Layer (SSL) and
Transport Layer Security (TLS) is enabled by default. You can disable password authentication
and SSL separately, as described in the next sections.
4 more annotations...
- SSL must be enabled (the default), to use client SSL authentication.
- To disable both password authentication and SSL (namely to disable all security),
  you should set the following system properties when you start the Java VM.
  com.sun.management.jmxremote.authenticate=false
  com.sun.management.jmxremote.ssl=false
- By default, the SSL, password, and access file properties are
  used for this connector.
- com.sun.management.jmxremote. ssl.need.client.auth
  
  If this property is true and the
  property com.sun.management.jmxremote.ssl is also true, then client authentication will be performed.
  
  true / false.
  Default is true.

SNMP Monitoring and Management - Java SE Monitoring and Management Guide

java.sun.com/...snmp.html - Preview

jvm JMX SNMP

Objects managed by SNMP are arranged in management information bases (MIBs).
To monitor a Java VM with SNMP you must first enable an
SNMP agent when you start the Java VM. You can enable the SNMP
agent for either a single-user environment or a multiple-user environment.

23 Apr 09

Mark Reinhold’s Blog

blogs.sun.com/jigsaw - Preview

modularization jdk OSGi jvm java programming language starred blog article feature toread

Its core has even already been implemented
within a Java virtual machine, namely that of Apache Harmony.

21 Apr 09

JDK 5.0 Java Virtual Machine Tool Interface (JVMTI)-related APIs & Developer Guides -- from Sun Microsystems

java.sun.com/...index.html - Preview

api jvm tools

Java HotSpot VM Options

java.sun.com/...vmoptions.jsp - Preview

jvm tuning athand

Categories of Java HotSpot VM Options
- Boolean options are turned on with -XX:+<option> and turned off with -XX:-<option>.
- Numeric options are set with -XX:<option>=<number>. Numbers can include 'm' or 'M' for megabytes, 'k' or 'K' for kilobytes, and 'g' or 'G' for gigabytes (for example, 32k is the same as 32768).
- String options are set with -XX:<option>=<string>, are usually used to specify a file, a path, or a list of commands
1 more annotations...
- -XX:ThreadStackSize=512

A Collection of JVM Options

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jvm tuning commands starred athand java

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The Programming Delusion: I am Visible but am I Accessible?

>: Type Checking - Java, Fast !

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Mark Reinhold’s Blog

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A Collection of JVM Options

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