Two-step verification.
.. -*- coding: utf-8; -*-
================
Java language.
================
.. contents::
:local:
Class version.
==============
========= ====== =====================
major minor Java platform version
========= ====== =====================
45 0x27 3 1.0
45 0x27 3 1.1
46 0x28 0 1.2
47 0x29 0 1.3
48 0x30 0 1.4
49 0x31 0 5.0
50 0x32 0 6.0
51 0x33 0 7
52 0x34 0 8
========= ====== =====================
where ``minor`` and ``major`` are value of 6 and 8 bytes in .class file::
0xCA, 0xFE, 0xBA, 0xBE, 0x00, minor, 0x00, major
Access modifiers.
=================
``public``:
* Public class is visible in other packages.
* Public field is visible everywhere (class must be public too).
``private``:
* Private variables or methods may be used only by an instance of the same
class that declares the variable or method
* A private feature may only be accessed by the class that owns the feature.
``protected``:
* Is available to all classes in the same package and also available to all
subclasses of the class that owns the protected feature.
* This access is provided even to subclasses that reside in a different
package from the class that owns the protected feature.
Default (ie, without any access modifier):
* It means that it is visible to all within a particular package.
``static``:
* Static means one per class, not one for each object no matter how many
instance of a class might exist. This means that you can use them without
creating an instance of a class.
* Static methods are implicitly final, because overriding is done based on
the type of the object, and static methods are attached to a class, not an
object.
* A static method in a superclass can be shadowed by another static method in
a subclass, as long as the original method was not declared final.
* You can't override a static method with a nonstatic method.
``final``:
* A final class can't be extended ie., final class may not be subclassed.
* A final method can't be overridden when its class is inherited.
* You can't change value of a final variable.
Exceptions
==========
A checked exception is some subclass of ``Exception`` (or ``Exception`` itself),
excluding class ``RuntimeException`` and its subclasses.
Unchecked exceptions are ``RuntimeException`` and any of its subclasses. Class
``Error`` and its subclasses also are unchecked. With an unchecked exception,
however, the compiler doesn't force client programmers either to catch the
exception or declare it in a throws clause.
Inner classes.
==============
Nested top-level classes.
-------------------------
If you declare a class within a class and specify the static modifier, the
compiler treats the class just like any other top-level class.
Any class outside the declaring class accesses the nested class with the
declaring class name acting similarly to a package. eg, outer.inner. Top-level
inner classes implicitly have access only to static variables. There can also
be inner interfaces. All of these are of the nested top-level variety.
Member classes.
---------------
Member inner classes are just like other member methods and member variables
and access to the member class is restricted, just like methods and variables.
This means a public member class acts similarly to a nested top-level class.
The primary difference between member classes and nested top-level classes is
that member classes have access to the specific instance of the enclosing
class.
Local classes.
--------------
Local classes are like local variables, specific to a block of code. Their
visibility is only within the block of their declaration. In order for the
class to be useful beyond the declaration block, it would need to implement a
more publicly available interface.
Because local classes are not members, the modifiers public, protected,
private, and static are not usable.
Anonymous classes.
------------------
Anonymous inner classes extend local inner classes one level further. As
anonymous classes have no name, you cannot provide a constructor.
Java performance.
=================
http://java.sun.com/performance/reference/whitepapers/5.0_performance.html
http://java.sun.com/performance/reference/whitepapers/6_performance.html
Java flags
==========
List of all supported flags by ``java``::
java -XX:+PrintFlagsFinal -version
Creating jar.
=============
::
$ jar cf myFile.jar *.class
$ jar cmf myManifestFile myFile.jar *.class
$ jar -cfe Main.jar foo.Main foo/Main.class
Profiling Java.
===============
::
$ java -Xprof com.vendor.product.Clazz
$ java -Xrunhprof:help
Debugging Java.
===============
Compile with ``-g`` to preserve source code information::
$ javac -g -cp $CLASSPATH -sourcepath $SRC_DIR -d $BUILD_DIR
To run Java program in debugger::
$ jdb -cp $CLASSPATH -sourcepath $SRC_DIR
To attach to Java application you should run application for Java 1.4::
$ java -Xdebug -Xrunjdwp:transport=dt_shmem,server=y,suspend=n,address=$PORT \
com.vendor.product.Clazz
for Java including & above 5::
$ java -agentlib:jdwp=transport=dt_shmem,server=y,suspend=n,address=$PORT \
com.vendor.product.Clazz
for Java 9 you need to set host::
$ java -agentlib:jdwp=transport=dt_shmem,server=y,suspend=n,address=*:$PORT \
com.vendor.product.Clazz
$ java -agentlib:jdwp=transport=dt_shmem,server=y,suspend=n,address=localhost:$PORT \
com.vendor.product.Clazz
and then attach with debugger::
$ jdb -attach $PORT
For ``transport`` use:
* ``dt_shmem`` for Windows
* ``dt_socket`` for Linux
Creating thread dump.
=====================
Dump current thread traces and memory statistic to stdout::
$ kill -QUIT $PID
If application started with ``nohup`` redirect output to selected file::
$ nohup java -cp ... com.app.Main >$NOHUP.log &
or check default ``nohup.out`` or ``$HOME/nohup.out`` file.
Alternatively use ``jstack`` utility for Java version 1.5 and above::
$ jstack -l $PID >$DUMP.out
``jstack`` should be runned from same user as probing application::
$ sudo -u $USER jstack -l $PID >$DUMP.out
.. NOTE:: Thread information preserved in heap dump and can be reviewed in
``visualvm``.
Creating heap dump from Java application.
=========================================
Find pid of Java process::
$ jps -l -v
Save dump to file::
$ jmap -dump:format=b,file=heap.hprof $PID
$ sudo -u $USR jcmd $PID GC.heap_dump heap.hprof
With ``live`` option only reachable objects are dumped::
$ jmap -dump:live,format=b,file=heap.hprof $PID
Creating heap dump on OutOfMemory error
=======================================
Run Java application with::
-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/path/to/heap.dump
https://docs.oracle.com/javase/7/docs/webnotes/tsg/TSG-VM/html/clopts.html
B.1.2 -XX:+HeapDumpOnOutOfMemoryError Option.
Debug class loading.
====================
To dump class loading and unloading to ``System.out`` add to ``java`` opts::
$ java -XX:+TraceClassLoading -XX:+TraceClassUnloading ...
$ java -verbose:class ...
To review loaded classes explore heap dump in ``visualvm`` (visit "Classes"
tab).
Decompile class file.
=====================
::
$ javap -v -p -c My.java
Graphical tools are available at:
http://jd.benow.ca/
JD-GUI is a standalone graphical utility that displays Java source codes of
“.class” files. You can browse the reconstructed source code with the JD-GUI
for instant access to methods and fields.
Find jar by class.
==================
* http://mvnrepository.com/search.html?query=PKG
* http://www.jarfinder.com
Set default Java in Debian.
===========================
::
$ update-java-alternatives -l
$ sudo update-java-alternatives -s java-1.8.0-openjdk-i386
or individually for each executable::
$ sudo update-alternatives --config java
Import SSL certificate.
=======================
Get cert with::
$ openssl s_client -connect promin-test.it.loc:433
-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----
or by opening URL in broswer and exporting in "Page info" ==> "Security" menu.
Call import utility with default ``changeit`` password::
$ keytool -importcert -file $YOUR.crt -keystore $JAVA_HOME/jre/lib/security/cacert -alias $ANY -storepass changeit
$ keytool -list -v -keystore $JAVA_HOME/jre/lib/security/cacert -storepass changeit
Import certificate system wide in Debian by (note, ``.crt`` extention is
mandatory)::
$ sudo mkdir /usr/share/ca-certificates/$ANY/ # don't mess with other certs
$ sudo cp /tmp/$YOUR.crt /usr/share/ca-certificates/$ANY/
$ sudo dpkg-reconfigure --force ca-certificates # check your cert in curses GUI!
$ sudo update-ca-certificates --fresh --verbose
Java EE versions.
=================
======= ======== ======== ========
Java EE Servlet JSP JSTL
======= ======== ======== ========
6 3.0 2.2 -
5 2.5 2.1 1.2
1.4 2.4 2.0 1.1
1.2 2.3 1.2 1.0
======= ======== ======== ========
To set servlet version check ``WEB-INF/web.xml``::
<web-app xmlns="http://java.sun.com/xml/ns/javaee" version="3.0">
See:
http://en.wikipedia.org/wiki/Java_EE_version_history
Java EE version history.
http://jcp.org/aboutJava/communityprocess/final/jsr315/index.html
Servlet 3.0 Specification
http://jcp.org/aboutJava/communityprocess/mrel/jsr154/index.html
Servlet 2.5 Specification
http://www.mularien.com/blog/2008/04/24/how-to-reference-and-use-jstl-in-your-web-application/
How to Reference and Use JSTL in your Web Application
Java interactive shell.
=======================
Just use Groovy. ``bsh`` is older alternative without code completion.
Object pretty-print
===================
::
org.apache.commons.lang3.builder.ToStringBuilder.reflectionToString()
org.apache.commons.lang3.builder.ReflectionToStringBuilder.toString()