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/
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