Java Fundamentals

Memo

• Modifier Types

  • private

  • default (no modifier): within package

  • protected: within package and sub-classes

  • public

• Special modifier

  • volatile: variables wouldn't be cached by threads, must be read / write within main memory.

  • transient: variables would be excluded during serialization / deserialization.

• Enum

  • Constants can be initialized with one or more attributes with different types.

    • Override hashcode / equals for hash collections and lambda grouping.

  • Constant name / toString, enum valueOf are mentioning the constant.

Java 8 Features

• Stream

  • Primitive Stream

    • Ex. IntStream

      • Method for iteration: range, rangeClosed

      • Using mapToInt(Class::getter).toArray() to assign to an int[]

      • Don't need to specify Comparator in min(), max()

  • Statistics method

    • Ex. average()

      • Using orElse() / getAsDouble() to set up a default value. (changing assignation from an OptionalDouble to a double)

    • summaryStatistics() encapsulates count, avg, sum, min, max.

  • findFirst() is deterministic, and findAny() is not.

  • Group By

    • One attribute

      • .collect(Collectors.groupingBy(Class::getterMethod))

    • Multiple attributes should be encapsulated into an tuple (DIY) object.

      • .collect(Collectors.groupingBy(object -> new Tuple(object.getter1(), object.getter2(), ..)

• Optional

• CompletableFuture

  • For callee to invoke other operations.

GC

• Goals

  • Shorten full GC time

  • Let fewer instances to get into to old generation

• Memory composition

  • Shared by threads

    • Heap (shared by threads, storing: non-primitive objects, class members, static variables)

      • Young Generation

        • Composition (suggested ratio - 8:1:1)

          • Eden

          • From Survivor

          • To Survivor

        • When Eden is full, trigger Minor GC to copy survivors into To Survivor, and release Eden, From Survivor. If To Survivor is full, instances get into the Old Generation.

        • Adjusted with -Xmn: (or separately with: -XX:NewSize, -XX:MaxNewSize)

      • Old Generation

        • When Old Generation is full, trigger Full GC to utilize Old Generation.

        • Old Generation space is: -Xmx subtracts with -Xmn

      • Heap is adjusted with: -Xms, -Xmx

    • Permanent (or called "Method area")

      • Storing classes, methods, constant pool

      • Adjusted with: -XX:PermSize, -XX:MaxPermSize

  • Owned by a thread, in Code Cache (or called "Native area") for threads.

    • Program counter

    • Natives

    • Stack (owned by each thread) to store:

      • Methods being called

      • Arguments being passed

      • Local primitive type variables, local references

    • Adjusted with: -Xss

• Algorithms

  • mark and sweep (used in old generation of heap)

  • copying (used in young generation of heap)

  • mark and compact (used in old generation of heap)

• Collectors

  • Serial

    • single threaded, mark and sweep (Effective but long Stop The World time)

  • Parallel

    • multi-threaded, mark and sweep (High throughput but long STW time)

  • CMS

    • concurrent mark and sweep (low STW, but consumes CPU)

    • steps

      • CMS inital mark (STW)

      • CMS concurrent mark

      • CMS remark (STW)

      • CMS concurrent sweep

  • G1

    • for low latency scenarios (replacement for CMS, suggested to use on machine with HEAP space of 6G up)

    • dividing heap into regions (1~32Mb)

• Tricks

  • To create a memory dump when the the OutOfMemory error occurs.

    • Add: -XX:+HeapDumpOnOutOfMemoryError

• Reference

  • Thread states
  • Java 記憶體組成與參數、回收判斷、回收演算法
  • GC Collector差異與概念作法
  • 如何优化Java GC
  • JVM 的内存管理与 GC 调优
  • Types of OutOfMemoryError
  • Tricks of the Trade: Tuning JVM Memory for Large-scale Services