TTL: Time to Live

gTLD Server: global top level dns server, like .com TLD server or .net TLD server. https://www.dnsknowledge.com/whatis/tld-name-server/

A record: maps domain to a specific IP

CNAME record: maps one name to another name

NX record: maps one name to a specific IP(stand for a dns merchine)

why do we need CNAME Record?

DDoS:

www.a.b.c 和 a.b.c在dns中是不同的

array.length length of array

String.length() number of unicode in the string

javac net/cheng/Add.java//compile net/cheng/Add.java and generate net/cheng/Add.class

javac -d tmp/ net/cheng/Add.java//compile net/cheng/Add.java and generate tm/net/cheng/Add.class, so -d specify the output dir

jar cvf ./tmp/add.jar net/cheng/Add.class

jar cvf apple.jar Test.class audio video //Test.class and two dirs audio and video are archived into alibaba.jar

jar cvf alibaba.jar com//com is the dir contains all classes

jar xvf alibaba.jar

java -cp .:tmp/* com.alibaba.Test//.:tmp/* means the java.class.path includes current dir . and all jar files in tmp/ ; Alternatively you can specify shell variaty: export CLASSPATH=.:tmp/*

Once you understand that classes are always referenced by their fully qualified name in the final bytecode, you must understand how the class code is actually loaded. This happens the first time an object of that class is created, or the first time a static member of the class is accessed. At this time, theClassLoadertries to locate the class and instantiate it. If it can't find the class, aClassNotFoundExceptionis thrown. To locate the class, theClassLoaderusually checks the paths listed in the$CLASSPATHenvironment variable.

All classes of Java SE API are in jre/lib/rt.jar and the extension jars are in jre/ext/lib/