java基礎篇：Runtime、System、cleaner、clone等基礎類

Runtime

描述運行時的狀態，也就是說在整個的JVM之中，Runtime類是唯一一個與JVM狀態有關的類，並且都會默認提供有一個默認的該類的實例化對象。

• 單例模式實現，Runtime.getRuntime()

• 保存的是JVM維護的系統信息

           Runtime runtime = Runtime.getRuntime();
          System.out.println("可用進程數：" + runtime.availableProcessors());
          System.out.println("最大內存：" + runtime.maxMemory()/1024/1024 + "M");
          System.out.println("總內存：" + runtime.totalMemory()/1024/1024+ "M");
          System.out.println("可用的內存：" + runtime.freeMemory()/1024/1024+ "M");
          runtime.gc();
  

System類

• 數組拷貝：public static void arraycopy()
• 獲取當前時間數組：System.currentTimeMillis()
• 進行垃圾回收：System.gc() ，就是調用的Runtime.gc()

cleaner

• 1.9之後提供了,java.lang.ref.Cleaner,當類被回收的時候，會調用的此方法，你可以在類中乾點什麼事情，比如喊一句“我要死了”、主要的功能是進行替代finialize()方法

• finialize()方法，1.9之前調用方式

  class Member{
      public Member(){
          System.out.println("構造方法，誕生類");
      }
  
      @Override
      protected void finalize() throws Throwable {
          System.out.println("回收方法");
          throw new Exception("我還要再活500年");
      }
  }
  
  public class test {
      public static void main(String[] args) throws InterruptedException {
          Member member = new Member();
          member = null;
          Runtime.getRuntime().gc();
      }
  }
  

• cleaner()方式，雖然比finialize麻煩了，但是效率更高了

  • CleaningExample implements AutoCloseableCleaningExample 是繼承於AutoCloseable
  • 新的監聽對象回收的方法，在需要在一個新的線程中執行，效率更高
  • 步驟如下：
    • 被監聽的類需要實現Runnable接口
    • 創建一個Cleaning，將類註冊到裏面去
    • 主線程中需要初始化 Cleaning類

class Member implements Runnable{
    public Member(){
        System.out.println("構造方法，誕生類");
    }

    @Override
    public void run() {
        System.out.println("清除方法");
    }
}

class MemberCleaning implements AutoCloseable{
    private static final Cleaner cleaner = Cleaner.create();
    private Member member;
    private Cleaner.Cleanable cleanable;
    public MemberCleaning(Member member){
        this.member = member; //創建新對象
        this.cleanable  = this.cleaner.register(this, this.member);
    }
    @Override
    public void close() throws Exception {
        this.cleanable.clean();//啓動線程
    }
}

public class test {
    public static void main(String[] args) throws InterruptedException {
    	Member member = new Member();
        try(MemberCleaning mc = new MemberCleaning(member)){

        }catch (Exception e){}
    }
}

clone克隆

• Object中的方法，所以，所有的類都可以被克隆，但是該類必須實現Cloneable接口

class Member implements Cloneable{
    private String name;
    private int age;

    public Member(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

    @Override
    public String toString() {
        return "Member{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

public class test {
    public static void main(String[] args) throws InterruptedException, CloneNotSupportedException {
        Member memberA = new Member("zhangsan",18);
        Member memberB = (Member) memberA.clone();
        System.out.println(memberA);
        System.out.println(memberB);

    }
}