mindlab

While developing an Eclipse based application I encountered a troubling error java.lang.OutOfMemoryError: PermGen space.

It doesn’t help to allocate more more heap memory with -Xmx.

This error is caused when the Garbage collector’s Permanent Generation fills up. The garbage collector first fills up the Young Generation and then a Minor Collection is done, some objects that were not collected are put in next generation, the Tenured Generation. After it fills up a Major Collection is performed. Then there is the third generation which is the Permanent Generation. Especially applications with a lot of classes may fill up this part of the heap.

To fix this problem the default permanent generation heap segment can be increased from 64M to something else. I used 128M which worked great.

-XX:MaxPermSize=128m

In a situation where the Mac OS X default Java is not the one you need for your app to use there is a way to force it to pick the version you want.

* Right click on your Eclipse.app in the Finder
* Select “Show Package Contents”
* Go to Contents/
* Open Info.plist in an editor
* Add the line
<string>-vm</string><string>/System/Library/Frameworks/
JavaVM.framework/Versions/1.6.0/Commands/java</string>

for setting Java 6

Otherwise you can change the default version of java as well in the Java properties. I have noticed a bug in it. I had the right setting, but it only took effect after I moved the versions around a bit . On one MacBook it still didn’t open the application with the right Java version so it had to be forced like described above.

Here follows an incomplete collection of a couple Mac OS X commands. Just open up Terminal.app, and try them out.

If you are familiar with UNIX you are lucky since most UNIX commands work.

ifconfig lists the IP address and a lot more network information. To keep the output shorter try ifconfig | grep inet.

locate finds files

pico opens a text editor in the shell

df displays how much free disk space is available

mkdir -p x/y/z creates all directories at once

pkgutil –pkgs list installed packages, to get details use pkgutil –pkg-info <package-name>

!$ refers to the last parameter and can be reused in the next command

find . -name *.xml | xargs grep MyString finds a string “MyString” in files in or below the current dir

find . -type f -print0 | xargs -0 chmod a-w changes write permissions recursively. Works if the files contain spaces slashes etc. Use -type d if you need to change the dirs instead.

Here are some additional commands: http://ss64.com/osx/

Non shell commands:

Capture a screen shot:

TCP/IP is a large collection of different communication protocols based upon the two original protocols TCP and IP:

• TCP (Transmission Control Protocol) communication between applications
• UDP (User Datagram Protocol) simple communication between applications
• IP (Internet Protocol) communication between computers
• ICMP (Internet Control Message Protocol) for errors and statistics
• DHCP (Dynamic Host Configuration Protocol) for dynamic addressing
• HTTP (Hyper Text Transfer Protocol) web browser to server
• SNMP (Simple Network Management Protocol) administration of networks
• NTP (Network Time Protocol) synchronize the time between computers
• SMTP (Simple Mail Transfer Protocol) transmission of e-mails
• IMAP (Internet Message Access Protocol) storing and retrieving e-mails
• POP (Post Office Protocol) downloading e-mails

TCP Uses a Fixed Connection and IP is Connection-Less. When an IP packet is sent from a computer, it arrives at an IP router. The IP router is responsible for “routing” the packet to the correct destination, directly or via another router.

TCP/IP is TCP and IP working together. TCP takes care of the communication between your application software (i.e. your browser) and your network software. IP takes care of the communication with other computers. TCP is responsible for breaking data down into IP packets before they are sent, and for assembling the packets when they arrive. IP is responsible for sending the packets to the correct destination.

Each computer must have a unique 4 byte  IP address. A domain name corresponds to an IP address and makes it easier to remember. It is translated by a DNS to an IP address.

A browser uses TCP/IP to access a server. A server uses TCP/IP to send HTML back to a browser. An e-mail program uses TCP/IP to connect to the Internet before sending and receiving e-mails using POP, SMTP and IMAP.

IP is at layer 3 in the OSI stack:

There are two sets of rules for subnetting TCP/IP networks. The original set of rules can be found in RFC 950, and the new set of rules can be found in RFC 1812.

A good analogy for IP addressing and packet forwarding (routing) is the snail mail analogy. Consider an IP packet to be an envelope containing data, and having an address on the front. Every TCP/IP-enabled network interface can be compared to a mailbox. Every mailbox (interface) has an IP address. The four bytes of an IP address can be compared to the state, city, street, and house number fields on the front of a snail mail envelope. A router in this analogy is a post office, that sorts and forwards mail based on the address on the envelope (packet header.) If the address is on the same street (based on the subnet mask,) the envelope (packet) is sent directly to the destination mailbox (interface) via local courier (Ethernet?). If the address is determined to be on another street, or in another city or state, the envelope (packet) is delivered via local courier (Ethernet?) to the street’s post office (router), where the postal workers (routing software) sort and forward mail based on established post office sorting procedures (routing tables.)

References:

http://www.w3schools.com/tcpip/tcpip_intro.asp

http://www.ipprimer.com/section.cfm

http://encyclopedia2.thefreedictionary.com/OSI+model

http://www.faqs.org/rfcs/rfc1812.html

Unfortunately decaf for your old Power PC Mac

For a PowerPC you can’t.

For Snow Leopard you just get it from Apple at  http://support.apple.com/downloads/Java_for_Mac_OS_X_10_5_Update_1