java API -- System类

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System类是我们一开始学习java就接触到的一个常用类，在这个类中封装了许多和系统底层有关的方法供我们调用。

所属包

首先，查看jdk我们可以知道，System类是位于java.lang包下的。因此，是无需导包就能直接使用的。

类描述

我们先看一段官方注解：

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/**
 * The <code>System</code> class contains several useful class fields
 * and methods. It cannot be instantiated.
 *
 * <p>Among the facilities provided by the <code>System</code> class
 * are standard input, standard output, and error output streams;
 * access to externally defined properties and environment
 * variables; a means of loading files and libraries; and a utility
 * method for quickly copying a portion of an array.
 *
 * @author  unascribed
 * @since   JDK1.0
 */

大意就是，System这个类一个基础工具类，它无法被实例化。

它主要用于标准输入、输出流的控制，以及数组的操作等。

构造函数

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/** Don't let anyone instantiate this class */
   private System() {
   }

由于构造函数的私有属性，因此System类是无法实例化的。

常用成员方法

输出方法

这里介绍两个方法，分别是：System.out.println()以及System.err.println()。

实际上，这两个方法都是通过System内的静态不可变成员变量out或err，并且它们都是PrintStream类的的对象引用，然后通过对象调用类中方法。

数组操作方法

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* @param      src      the source array.
    * @param      srcPos   starting position in the source array.
    * @param      dest     the destination array.
    * @param      destPos  starting position in the destination data.
    * @param      length   the number of array elements to be copied.
    * @exception  IndexOutOfBoundsException  if copying would cause
    *               access of data outside array bounds.
    * @exception  ArrayStoreException  if an element in the <code>src</code>
    *               array could not be stored into the <code>dest</code> array
    *               because of a type mismatch.
    * @exception  NullPointerException if either <code>src</code> or
    *               <code>dest</code> is <code>null</code>.
    */
   public static native void arraycopy(Object src,  int  srcPos,
                                       Object dest, int destPos,
                                       int length);

从源数组src向目的数组dest，复制给定长度length的数据。

退出jvm方法

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/**
    * Terminates the currently running Java Virtual Machine. The
    * argument serves as a status code; by convention, a nonzero status
    * code indicates abnormal termination.
    * <p>
    * This method calls the <code>exit</code> method in class
    * <code>Runtime</code>. This method never returns normally.
    * <p>
    * The call <code>System.exit(n)</code> is effectively equivalent to
    * the call:
    * <blockquote><pre>
    * Runtime.getRuntime().exit(n)
    * </pre></blockquote>
    *
    * @param      status   exit status.
    * @throws  SecurityException
    *        if a security manager exists and its <code>checkExit</code>
    *        method doesn't allow exit with the specified status.
    * @see        java.lang.Runtime#exit(int)
    */
   public static void exit(int status) {
       Runtime.getRuntime().exit(status);
   }

方法描述为：终止正在运行的jvm，并返回一个状态码。依照惯例：返回‘0’，代表正常退出 ；其他值，代表异常退出。

垃圾回收方法

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/**
     * Runs the garbage collector.
     * <p>
     * Calling the <code>gc</code> method suggests that the Java Virtual
     * Machine expend effort toward recycling unused objects in order to
     * make the memory they currently occupy available for quick reuse.
     * When control returns from the method call, the Java Virtual
     * Machine has made a best effort to reclaim space from all discarded
     * objects.
     * <p>
     * The call <code>System.gc()</code> is effectively equivalent to the
     * call:
     * <blockquote><pre>
     * Runtime.getRuntime().gc()
     * </pre></blockquote>
     *
     * @see     java.lang.Runtime#gc()
     */
    public static void gc() {
        Runtime.getRuntime().gc();
    }

调用jvm的垃圾回收机制，去尽可能的回收系统资源。一般与Object类的finalize()方法配套使用

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/**
    * Called by the garbage collector on an object when garbage collection
    * determines that there are no more references to the object.
    * A subclass overrides the {@code finalize} method to dispose of
    * system resources or to perform other cleanup.
    * <p>
 protected void finalize() throws Throwable { }

流程是：

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public class Test{
    public static void main(String[] args){
        new Student(); //创建匿名对象，方便gc回收
        System.gc();   //调用jvm回收机制
    }
}

class Student{
	@override
    void finalize(){
        System.out.println("在这里完成一些收尾工作");
    }
}

运行结果是：可能打印出“在这里完成一些收尾工作”。因为gc不能保证每次都能回收Student资源（the Java Virtual Machine has made a best effort to reclaim space from all discarded）。