Java Series: Basics Q&A Part 7

Q31. How to defend Java Application from Inject Attack?

Select * from use_info where username = “input_usr_name” and password = “input_pwd”
“ or ""="
Select * from use_info where username = “input_usr_name” and password = “” or “” = “”
ll input_file_name
input_file_name;rm -rf /*

Q32. How to write secure Java code?

  • Usage of early version of JDK & Applet.
  • Hash collision attack. Especially pay attention to computational heavy tasks, like encryption/decryption/image processing etc.
  • Services that take user input, like unzip files (Zip Bomb)
  • DB connection, file descriptor leak or even reentrant lock. Make sure all resources can be released under all scenarios.
if (a + b < c) { // … }
try {
// ..
} catch (Exception e) {
throw new RuntimeException(hostname + port + “ doesn’t response”);

Q33. How to diagnose backend services slowing down?

  1. Check error logs.
  2. Monitor JVM. Any Full GC or Minor GC is getting longer?
  3. Profiling.
  4. Check CPU usage.
  5. Use vmstat to check the number of context switches.

Q34. Does Lambda always slow down Java program?

public void testMethod() {
// Put your benchmark code here.
  1. Make sure the code is warmed up (the code is already compiled into native code instead of JIT).
  2. Avoid JVM dead code elimination.
  3. Avoid constant folding.
  4. Avoid false sharing.

Q35. How does JVM optimize Java code?

  • print compilation details:
  • Print more compilation details
-XX:UnlockDiagnosticVMOptions -XX:+LogCompilation -XX:LogFile=<your_file_path>
  • Print function inline
  • Tweak JIT threshold
  • Tweak JVM counter decay switch
  • Update size of Code Cache. JIT code will be stored in code cache.

Q36. MySQL isolation level & Optimistic/Pessimistic Lock?

  1. Read uncommitted. User is able to see other uncommitted changes. You might see intermittent state.
  2. Read committed. User is able to see committed changes, no intermittent changes. This doesn’t guarantee second read would get the same data, Phantom Read might happen.
  3. Repeatable reads. The same data will be consistent during multiple reads, this is the default level of isolation in MySQL.
  4. Serializable. This is the highest level of isolation. There will be a shared lock for reading and update needs exclusive lock.




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