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Written by Oleksandr Gavenko (AKA gavenkoa), compiled on 2019-01-03 from rev c44e4331713c.

Java language.

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 class is visible in other packages.
  • Public field is visible everywhere (class must be public too).


  • 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.


  • 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.


What you get by default ie, without any access modifier.

  • It means that it is visible to all within a particular package.


  • 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.


  • 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.


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.

64-bit problem.

Which version of Java should I download for my 64-bit Windows operating system?
Java SE 6 Release Notes Supported System Configurations

Java performance.

http://java.sun.com/performance/reference/whitepapers/5.0_performance.html http://java.sun.com/performance/reference/whitepapers/6_performance.html

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 firstly must run application with (use dt_shmem for Windows and dt_socket for Linux):

$ java -Xdebug -Xrunjdwp:transport=dt_shmem,server=y,suspend=n,address=$PORT \

and then attach with debugger:

$ jdb -attach $PORT

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


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.dump $PID

With live option only reachable objects are dumped:

$ jmap -dump:live,format=b,file=heap.dump $PID

Creating heap dump on OutOfMemory error.

Run Java application with:

-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/path/to/heap.dump

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:

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.

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


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">


Java EE version history.
Servlet 3.0 Specification
Servlet 2.5 Specification
How to Reference and Use JSTL in your Web Application

Java interactive shell.

Just use Groovy. bsh is older alternative without code completion.