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Java

Most of these notes are from the book Java Projects by Peter Verhas.

JConsole over SSH with SOCKS Proxy

This is from here: Create the SSH socks proxy locally on some free port (e.g. 7777):

ssh -fN -D 7777 user@firewalled-host

Run JConsole by specifying the SOCKS proxy (e.g. localhost:7777) and the address for the JMX server (e.g. localhost:2147)

jconsole -J-DsocksProxyHost=localhost -J-DsocksProxyPort=7777 service:jmx:rmi:///jndi/rmi://localhost:2147/jmxrmi -J-DsocksNonProxyHosts=

JShell

  • /list -start - shows modules imported at startup.
  • /edit <number> - edit that line in a new window.
  • /set editor "vi" - use vi instead of the default graphical edit pad.
  • /save abc.java - save current buffer to file.
  • /load abc.java - load from file into shell.
  • /-1 - execute last snippet.
  • /1 - execute first snippet.
  • /drop N - drop Nth snippet.
  • /vars - show only variables that were defined in snippets.
  • /types - show only classes that were defined in snippets.

Javap

  • javap TestDecompile.class - decompile .class file to human readable format. Does not show content of methods though.

  • javap -c TestDecompile.class - show jvm bytecode in human readable -form, including methods.

Jar files

These are zip files that have a META-INF folder with a Manifest.mf file inside.

To create a jar file:

jar -cf hello.jar HelloWorld.class

And to run:

java -cp hello.jar HelloWorld

To see contents of a jar:

jar -tf hello.jar

Runtime/debugging

  • jps - Shows all runnning java processes.

  • jvisualvm - If you have it, it shows the java processes on the system with details on threads, profiler etc.

  • -Xagentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=7896 Use these args to start a process in debugging mode.

    • agentlib=jdwp - Load the jdwp agent, for debugging.

    • transport=dt_socket - for connecting a debugging client over the network.

    • server=y - this one is the server half of the debugging.

    • suspend=y - don't start executing until a client debugger attaches.

    • address=7896 - port we listen on.

Maven

  • Expected directory structure: Java files are in src/main/java as well as src/test/java. Resource files are under src/main/resources and src/test/resources.

  • mvn archetype:generate: Generates a skeleton of a project based on your inputs (package name, versioning, project name, etc).

  • Edit pom.xml and set the jdk version there.. the default may be old. Java 9 onwaards doesn't generate the .class format. You should have this added to the default .xml file:

<build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.8.0</version>
        <configuration>
            <source>1.11</source>
            <target>1.11</target>
            <release>11</release>
        </configuration>
      </plugin>
    </plugins>
  </build>
  • mvn package - compile, test, bundle.

Interfaces

  • All fields (i.e. class variables) default to static final. All methods default to public.

Field types

  • static - One instance per class, not per object.

  • final - Initialized exactly once, either where declared or

  • somewhere in the constructor. Immutable after that.

  • transient - Do not serialize/deserialize this field.

  • volatile - Treat as a field that may be accessed by multiple threads. If not volatile, the value may be stored in a processor cache or registry for faster access.

Things to find out later

  • See how to create a call graph in eclipse.

  • final keyword?

    • Answer: when used with a variable, it (a) blocks reassignment, and (b) limits existence to that block. e.g. a final String tmp = orig inside a for loop cannot be reassigned with a different value. The value does not exist outside the block it is defined in. Good practice to declare this often as it means a variable is guaranteed not to change in value.
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