.. -*- 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()