理解 Java 的 GC 与 幽灵引用

         Java 中一共有 4 种类型的引用 : StrongReference、 SoftReference、 WeakReference 以及 PhantomReference (传说中的幽灵引用 呵呵),
这 4 种类型的引用与 GC 有着密切的关系,  让我们逐一来看它们的定义和使用场景 :

        1. Strong Reference
       
        StrongReference 是 Java 的默认引用实现,  它会尽可能长时间的存活于 JVM 内， 当没有任何对象指向它时 GC 执行后将会被回收

@Test
public void strongReference() {
    Object referent = new Object();

    /**
     * 通过赋值创建 StrongReference
     */
    Object strongReference = referent;

    assertSame(referent, strongReference);

    referent = null;
    System.gc();

    /**
     * StrongReference 在 GC 后不会被回收
     */
    assertNotNull(strongReference);
}

	@Test
	public void strongReference() {
		Object referent = new Object();
		
		/**
		 * 通过赋值创建 StrongReference 
		 */
		Object strongReference = referent;
		
		assertSame(referent, strongReference);
		
		referent = null;
		System.gc();
		
		/**
		 * StrongReference 在 GC 后不会被回收
		 */
		assertNotNull(strongReference);
	}
	

        2. WeakReference & WeakHashMap

WeakReference， 顾名思义,  是一个弱引用,  当所引用的对象在 JVM 内不再有强引用时, GC 后 weak reference 将会被自动回收

@Test
public void weakReference() {
    Object referent = new Object();
    WeakReference weakRerference = new WeakReference(referent);   
  
    assertSame(referent, weakRerference.get());   
       
    referent = null;   
    System.gc();   
       
    /**  
     * 一旦没有指向 referent 的强引用, weak reference 在 GC 后会被自动回收  
     */  
    assertNull(weakRerference.get());   
}   	@Test
	public void weakReference() {
		Object referent = new Object();
		WeakReference weakRerference = new WeakReference(referent);
	
		assertSame(referent, weakRerference.get());
		
		referent = null;
		System.gc();
		
		/**
		 * 一旦没有指向 referent 的强引用, weak reference 在 GC 后会被自动回收
		 */
		assertNull(weakRerference.get());
	}
	
WeakHashMap 使用 WeakReference 作为 key， 一旦没有指向 key 的强引用, WeakHashMap 在 GC 后将自动删除相关的 entry


Java代码 
@Test
public void weakHashMap() throws InterruptedException {
    Map weakHashMap = new WeakHashMap();
    Object key = new Object();
    Object value = new Object();
    weakHashMap.put(key, value);

    assertTrue(weakHashMap.containsValue(value));

    key = null;
    System.gc();

    /**
     * 等待无效 entries 进入 ReferenceQueue 以便下一次调用 getTable 时被清理
     */
    Thread.sleep(1000);

    /**
     * 一旦没有指向 key 的强引用, WeakHashMap 在 GC 后将自动删除相关的 entry
     */
    assertFalse(weakHashMap.containsValue(value));
}	@Test
	public void weakHashMap() throws InterruptedException {
		Map weakHashMap = new WeakHashMap();
		Object key = new Object();
		Object value = new Object();
		weakHashMap.put(key, value);
	
		assertTrue(weakHashMap.containsValue(value));
		
		key = null;
		System.gc();
		
		/**
		 * 等待无效 entries 进入 ReferenceQueue 以便下一次调用 getTable 时被清理
		 */
		Thread.sleep(1000);
		
		/**
		 * 一旦没有指向 key 的强引用, WeakHashMap 在 GC 后将自动删除相关的 entry
		 */
		assertFalse(weakHashMap.containsValue(value));
	}
	
        3. SoftReferenceSoftReference 于 WeakReference 的特性基本一致， 最大的区别在于 SoftReference 会尽可能长的保留引用直到 JVM 内存不足时才会被回收(虚拟机保证), 这一特性使得 SoftReference 非常适合缓存应用


Java代码 
@Test
public void softReference() {
    Object referent = new Object();
    SoftReference softRerference = new SoftReference(referent);   
  
    assertNotNull(softRerference.get());   
       
    referent = null;   
    System.gc();   
       
    /**  
     *  soft references 只有在 jvm OutOfMemory 之前才会被回收, 所以它非常适合缓存应用  
     */  
    assertNotNull(softRerference.get());   
}   	@Test
	public void softReference() {
		Object referent = new Object();
		SoftReference softRerference = new SoftReference(referent);
	
		assertNotNull(softRerference.get());
		
		referent = null;
		System.gc();
		
		/**
		 *  soft references 只有在 jvm OutOfMemory 之前才会被回收, 所以它非常适合缓存应用
		 */
		assertNotNull(softRerference.get());
	}

	
        4. PhantomReference        作为本文主角， Phantom Reference(幽灵引用) 与 WeakReference 和 SoftReference 有很大的不同,  因为它的 get() 方法永远返回 null, 这也正是它名字的由来


Java代码 
@Test
public void phantomReferenceAlwaysNull() {
    Object referent = new Object();
    PhantomReference phantomReference = new PhantomReference(referent, new ReferenceQueue());   
       
    /**  
     * phantom reference 的 get 方法永远返回 null   
     */  
    assertNull(phantomReference.get());   
}   	@Test
	public void phantomReferenceAlwaysNull() {
		Object referent = new Object();
		PhantomReference phantomReference = new PhantomReference(referent, new ReferenceQueue());
		
		/**
		 * phantom reference 的 get 方法永远返回 null 
		 */
		assertNull(phantomReference.get());
	}

	
         诸位可能要问, 一个永远返回 null 的 reference 要来何用,  请注意构造 PhantomReference 时的第二个参数 ReferenceQueue(事实上 WeakReference & SoftReference 也可以有这个参数)，PhantomReference 唯一的用处就是跟踪 referent  何时被 enqueue 到 ReferenceQueue 中.     5. RererenceQueue当一个 WeakReference 开始返回 null 时， 它所指向的对象已经准备被回收， 这时可以做一些合适的清理工作.   将一个 ReferenceQueue 传给一个 Reference 的构造函数， 当对象被回收时， 虚拟机会自动将这个对象插入到 ReferenceQueue 中， WeakHashMap 就是利用 ReferenceQueue 来清除 key 已经没有强引用的 entries.


Java代码 
@Test
public void referenceQueue() throws InterruptedException {
    Object referent = new Object();
    ReferenceQueue referenceQueue = new ReferenceQueue();   
    WeakReference weakReference = new WeakReference(referent, referenceQueue);   
       
    assertFalse(weakReference.isEnqueued());   
    Referenceextends Object> polled = referenceQueue.poll();   
    assertNull(polled);   
       
    referent = null;   
    System.gc();   
  
    assertTrue(weakReference.isEnqueued());   
    Referenceextends Object> removed = referenceQueue.remove();   
    assertNotNull(removed);   
}  	@Test
	public void referenceQueue() throws InterruptedException {
		Object referent = new Object();		
		ReferenceQueue referenceQueue = new ReferenceQueue();
		WeakReference weakReference = new WeakReference(referent, referenceQueue);
		
		assertFalse(weakReference.isEnqueued());
		Reference polled = referenceQueue.poll();
		assertNull(polled);
		
		referent = null;
		System.gc();

		assertTrue(weakReference.isEnqueued());
		Reference removed = referenceQueue.remove();
		assertNotNull(removed);
	}

6.  PhantomReference  vs WeakReferencePhantomReference  有两个好处， 其一， 它可以让我们准确地知道对象何时被从内存中删除， 这个特性可以被用于一些特殊的需求中(例如 Distributed GC，  XWork 和 google-guice 中也使用 PhantomReference 做了一些清理性工作). 其二， 它可以避免 finalization 带来的一些根本性问题, 上文提到 PhantomReference 的唯一作用就是跟踪 referent 何时被 enqueue 到 ReferenceQueue 中,  但是 WeakReference 也有对应的功能, 两者的区别到底在哪呢 ?这就要说到 Object 的 finalize 方法, 此方法将在 gc 执行前被调用, 如果某个对象重载了 finalize 方法并故意在方法内创建本身的强引用,  这将导致这一轮的 GC 无法回收这个对象并有可能引起任意次 GC， 最后的结果就是明明 JVM 内有很多 Garbage 却 OutOfMemory， 使用 PhantomReference 就可以避免这个问题， 因为 PhantomReference 是在 finalize 方法执行后回收的，也就意味着此时已经不可能拿到原来的引用,  也就不会出现上述问题,  当然这是一个很极端的例子, 一般不会出现. 7. 对比taken from 




Soft vs Weak vs Phantom References

Type
Purpose
Use
When GCed
Implementing Class


Strong Reference
An ordinary reference. Keeps objects alive as long as they are referenced.
normal reference.
Any object not pointed to can be reclaimed.
default

Soft Reference
Keeps objects alive provided there’s enough memory.
to keep objects alive even after clients have removed their references (memory-sensitive caches), in case clients start asking for them again by key.
After a first gc pass, the JVM decides it still needs to reclaim more space.
java.lang.ref.SoftReference 

Weak Reference
Keeps objects alive only while they’re in use (reachable) by clients.
Containers that automatically delete objects no longer in use.
After gc determines the object is only weakly reachable
java.lang.ref.WeakReference java.util.WeakHashMap 

Phantom Reference
Lets you clean up after finalization but before the space is reclaimed (replaces or augments the use offinalize())
Special clean up processing
After finalization.
java.lang.ref.PhantomReference8. 小结       一般的应用程序不会涉及到 Reference 编程， 但是了解这些知识会对理解 GC 的工作原理以及性能调优有一定帮助,   在实现一些基础性设施比如缓存时也可能会用到， 希望本文能有所帮助.        王政 于 2009，6，3                                   
            
            
              
			  
			  
				
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					{
						var tpl   = '<div class="Blog_right1_8">';
							tpl  +=   '<div class="Blog_right_img1"><a href="' +data.info.url+ '" >' + data.info.header + '</a></div>';
							tpl  +=	  '<div class="Blog_right_font1">';
							tpl  +=       '<p class="Blog_p5"><span><a href="' +data.info.url+ '" >' + data.info.username + '</a></span>' + data.info.dateline + '</p>';
							tpl  +=       '<p class="Blog_p6">' + data.info.content + '</p>';
							tpl  +=		  '<div class="div1"><a href="javascript:void(0);" class="Blog_a10"  toid=' + data.info.fuid + ' bid=' + data.info.bid + ' cid=' + data.info.cid + '>回复</a> | 　<a class="comment_del_mark" style="cursor:pointer" url="' + data.info.delurl + '">删除</a>　| 　<a href="javascript:showJuBao(\'/blog/ViewReport.html\','+data.info.cid+')" class="box_report" cid="' + data.info.cid + '">举报</a></div>';
							tpl  +=		  '<div class="z_move_comment" style="display:none"></div>';
							tpl  +=    '</div><div class="Blog_line1"></div></div>';
							$('.Blog_tit3:first').after(tpl);
							$('#reply').val('文明上网，理性发言...');
					}
					else if(data.code == -1)
					{
						showErrorMsg(data.info , '消息提示'); 
			        }
                },
			    error: function(){//请求出错处理
                        
                }
		    });
			
		});
		//底部评论重置
		$('#reset_comment').click(function(){
			$('#reply').val('文明上网，理性发言...');
		});
		//取消回复
		$('#qx_comment').live('click' , function(){
			$('.z_move_comment').html('').hide();
		});


		$('#rmsg, #reply').live({
		    focus:function(){
			    if($(this).val() == '文明上网，理性发言...'){
			        $(this).val('');
			    }
			},
			blur:function(){
		        if($(this).val() == ''){
			        $(this).val('文明上网，理性发言...');
			    }
			}
		});
		//删除留言确认
		$('.comment_del_mark').live('click' , function(){
			var url = $(this).attr('url');
			asyncbox.confirm('删除留言','确认', function(action){
				if(action == 'ok')
				{
					location.href = url;
				}
			});
		});
		//删除时间确认
		$('.del_article_id').click(function(){
			var delurl = $(this).attr('delurl');
			asyncbox.confirm('删除文章','确认', function(action){
				if(action == 'ok')
				{
					location.href = delurl;
				}
			});
		});
		/*
		//字数限制
		$('#rmsg, #reply').live('keyup', function(){
			var id = $(this).attr('id');
			var left = Util.calWbText($(this).val(), 500);
			var eid = '#errmsg';
			
			if(id == 'reply') eid =  '#rerrmsg';
			if (left >= 0)
		        $(eid).html('您还可以输入<span>' + left + '</span>字');
		    else
		        $(eid).html('<font color="red">您已超出<span>' + Math.abs(left) + '</span>字 </font>');
		});
		*/
		//加载表情
	    $('#face').qqFace({id : 'facebox1', assign : 'reply', path : '/image/qqface/'});
	    $('#mface').qqFace({id : 'facebox', assign : 'rmsg', path:'/image/qqface/'});
	    
		/*
		$('#class_one_id').change(function(){
			alert(123213);
			var id = parseInt($(this).val() , 10);
			if(id == 0) return false;
			$('.hidden_son_class span').each(function( index , dom ){
				if( dom.attr('cid')  == id )
				{
				}
			});
		});
		*/
		//转载文章
		var turn_url = "/blog/viewClassPart.html";
		$('#repost_bar').click(function(){
			if(isOnLine == '' )
			{
				//showErrorMsg('登录之后才能进行此操作' , '消息提示');
				showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
				return false;
			}
			var id = $(this).attr('bid');
			asyncbox.open({
				id  : 'turn_class_thickbox',
				url : turn_url,
				title : '转载文章',
				width : 330,
				height : 131,
				scroll : 'no',
				data : {
					'id'	 : id
				},
				callback : function(action){
					if(action == 'close'){
						$.cover(false);
					}
				}
			});
		});
		/*
	    //转发文章
	    $('#repost_bar').live('click' , function(){
			if(isOnLine == '' )
			{
				//showErrorMsg('登录之后才能进行此操作' , '消息提示');
				showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
				return false;
			}
	    	var bid  = $(this).attr('bid');
	    	var url  = $(this).attr('url');
	    	asyncbox.confirm('转载文章','确认', function(action){
	    	  if(action == 'ok'){
	    	    $.ajax({
				type:"POST",
				url:url,	
				data: {
					'bid' : bid 
				},
				dataType: 'json',
				success:function(msg){
				  if(msg.error == 0){
				    showSucceedMsg('转发成功！', '消息提示');
				  }else if(msg.error == 1){
				    //location.href = '/index.php?r=site/login';
					showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
				  }else{
				    showErrorMsg(msg.error_content, '消息提示');
			      }
				}
			  });
	    	  }
	    	});

		});
		*/

	});




SyntaxHighlighter.autoloader(
	'applescript			/highlight/scripts/shBrushAppleScript.js',
	'actionscript3 as3		/highlight/scripts/shBrushAS3.js',
	'bash shell				/highlight/scripts/shBrushBash.js',
	'coldfusion cf			/highlight/scripts/shBrushColdFusion.js',
	'cpp c					/highlight/scripts/shBrushCpp.js',
	'c# c-sharp csharp		/highlight/scripts/shBrushCSharp.js',
	'css					/highlight/scripts/shBrushCss.js',
	'delphi pascal			/highlight/scripts/shBrushDelphi.js',
	'diff patch pas			/highlight/scripts/shBrushDiff.js',
	'erl erlang				/highlight/scripts/shBrushErlang.js',
	'groovy					/highlight/scripts/shBrushGroovy.js',
	'java					/highlight/scripts/shBrushJava.js',
	'jfx javafx				/highlight/scripts/shBrushJavaFX.js',
	'js jscript javascript	/highlight/scripts/shBrushJScript.js',
	'perl pl				/highlight/scripts/shBrushPerl.js',
	'php					/highlight/scripts/shBrushPhp.js',
	'text plain				/highlight/scripts/shBrushPlain.js',
	'py python				/highlight/scripts/shBrushPython.js',
	'ruby rails ror rb		/highlight/scripts/shBrushRuby.js',
	'scala					/highlight/scripts/shBrushScala.js',
	'sql					/highlight/scripts/shBrushSql.js',
	'vb vbnet				/highlight/scripts/shBrushVb.js',
	'xml xhtml xslt html	/highlight/scripts/shBrushXml.js'
);
SyntaxHighlighter.all();


function code_hide(id){
	var code = document.getElementById(id).style.display;
	if(code == 'none'){
		document.getElementById(id).style.display = 'block';
	}else{
		document.getElementById(id).style.display = 'none';
	}
}



 lastScrollY=0;
 function heartBeat(){
 var diffY;
 if (document.documentElement && document.documentElement.scrollTop)
     diffY = document.documentElement.scrollTop;
 else if (document.body)
     diffY = document.body.scrollTop
 else
     {/*Netscape stuff*/}
 percent=.1*(diffY-lastScrollY);
 if(percent>0)percent=Math.ceil(percent);
 else percent=Math.floor(percent);
 document.getElementById("full").style.top=parseInt(document.getElementById("full").style.top)+percent+"px";
 lastScrollY=lastScrollY+percent;
 if(lastScrollY<200)
 {
 document.getElementById("full").style.display="none";
 }
 else
 {
 document.getElementById("full").style.display="block";
 }
 }
 var gkuan=document.body.clientWidth;
 var ks=(gkuan-960)/2-30;
 suspendcode="<div id=\"full\" style='right:-30px;POSITION:absolute;TOP:500px;z-index:100;width:26px; height:86px;cursor:pointer;'><a href=\"javascript:void(0)\" onclick=\"window.scrollTo(0,0);\"><img src=\"\/image\/top.png\" /></a></div>"
 document.write(suspendcode);
 window.setInterval("heartBeat()",1);
 
  
  
    关于我们 | 关于IT168 | 联系方式 | 广告合作 | 法律声明 | 免费注册
      Copyright  2001-2010 ChinaUnix.net All Rights Reserved 北京皓辰网域网络信息技术有限公司. 版权所有 
      感谢所有关心和支持过ChinaUnix的朋友们
        16024965号-6 
      
    
  



//全局错误提示弹出框
function showErrorMsg(content, title, url){
	var url = url || '';
	var title = title || '消息提示';
	var html = '';
	html += '<div class="HT_layer3_1 HT_layer3_2"><ul><li><p><span class="login_span1"></span>' + content + '</p></li>';
	if(url == '' || url.length == 0){
		html += '<li class="HT_li1"><input type="button" class="HT_btn2"  onclick=\'close_windows("error_msg")\'></li>';
	} else {
		html += '<li class="HT_li1"><input type="button" class="login_btn1" onclick="location.href=\'' + url + '\'"></li>';
	}
	html += '</ul></div>';
	$.cover(true);
	   asyncbox.open({
		 id: 'error_msg',
		 title : title,
		 html : html,
		 'callback' : function(action){
			 if(action == 'close'){
				 $.cover(false);
			 }
		 }
	});
}

//全局正确提示
function showSucceedMsg(content, title , url ){
	var url = url || '';
	var title = title || '消息提示';
	var html = '';
	html += '<div class="HT_layer3_1 HT_layer3_2"><ul><li><p><span class="login_span2"></span>' + content + '</p></li>';
	if(url == '' || url.length == 0){
		html += '<li class="HT_li1"><input type="button" class="HT_btn2"  onclick=\'close_windows("error_msg")\'></li>';
	} else {
		html += '<li class="HT_li1"><input type="button" class="HT_btn2" onclick="location.href=\'' + url + '\'"></li>';
	}
	html += '</ul></div>';
	$.cover(true);
	asyncbox.open({
		 id: 'error_msg',
		 title : title,
		 html : html,
		 'callback' : function(action){
			 if(action == 'close'){
				 $.cover(false);
			 }
		 }
	});
}

//关闭指定id的窗口
function close_windows(id){
	$.cover(false);
	$.close(id);
}


//公告
var tID;
var tn;                        // 高度
var nStopTime = 5000;        // 不同行间滚动时间隔的时间，值越小，移动越快
var nSpeed = 50;            // 滚动时，向上移动一像素间隔的时间，值越小，移动越快
var isMove = true;
var nHeight = 25;
var nS = 0;
var nNewsCount = 3;

/**
 * n 用于表示是否为第一次运行
 **/
function moveT(n)
{
    clearTimeout(tID)
    var noticev2 = document.getElementById("noticev2")
    nS = nSpeed;

    // 只在第一次调用时运行，初始化环境（有没有参数）
    if (n)
    {
        // 设置行高
        noticev2.style.lineHeight = nHeight + "px";
        // 初始化显示位置
        tn = 0;
        // 刚进入时在第一行停止时间
        nS = nStopTime;
    }

    // 判断鼠标是否指向层
    if (isMove)
    {
        // 向上移动一像素
        tn--;
        // 如果移动到最下面一行了，则移到顶行
        if (Math.abs(tn) == nNewsCount * nHeight)
        {
            tn = 0;
        }
        // 设置位置
        noticev2.style.marginTop = tn + "px";
        // 完整显示一行时，停止一段时间
        if (tn % nHeight == 0)
        {
            nS = nStopTime;
        }
    }

    tID = setTimeout("moveT()", nS);
}
moveT(1);    // 此处可以传入任何参数


//  var _gaq = _gaq || [];
//  _gaq.push(['_setAccount', 'UA-20237423-2']);
//  _gaq.push(['_setDomainName', '.chinaunix.net']);
//  _gaq.push(['_trackPageview']);
//
//  (function() {
//    var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
//    ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
//    var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
//  })();



var _bdhmProtocol = (("https:" == document.location.protocol) ? " https://" : " http://");
document.write(unescape("%3Cscript src='" + _bdhmProtocol + "hm.baidu.com/h.js%3F0ee5e8cdc4d43389b3d1bfd76e83216b' type='text/javascript'%3E%3C/script%3E"));

function link(t){
     var href= $(t).attr('href');
     href+="?url="+encodeURIComponent(location.href);
     $(t).attr('href',href);
    //setCookie("returnOutUrl", location.href, 60, "/");
}
    WeakReference weakReference = new WeakReference(referent, referenceQueue);   
       
    assertFalse(weakReference.isEnqueued());   
    Referenceextends Object> polled = referenceQueue.poll();   
    assertNull(polled);   
       
    referent = null;   
    System.gc();   
  
    assertTrue(weakReference.isEnqueued());   
    Referenceextends Object> removed = referenceQueue.remove();   
    assertNotNull(removed);   
}  	@Test
	public void referenceQueue() throws InterruptedException {
		Object referent = new Object();		
		ReferenceQueue referenceQueue = new ReferenceQueue();
		WeakReference weakReference = new WeakReference(referent, referenceQueue);
		
		assertFalse(weakReference.isEnqueued());
		Reference polled = referenceQueue.poll();
		assertNull(polled);
		
		referent = null;
		System.gc();

		assertTrue(weakReference.isEnqueued());
		Reference removed = referenceQueue.remove();
		assertNotNull(removed);
	}

6.  PhantomReference  vs WeakReferencePhantomReference  有两个好处， 其一， 它可以让我们准确地知道对象何时被从内存中删除， 这个特性可以被用于一些特殊的需求中(例如 Distributed GC，  XWork 和 google-guice 中也使用 PhantomReference 做了一些清理性工作). 其二， 它可以避免 finalization 带来的一些根本性问题, 上文提到 PhantomReference 的唯一作用就是跟踪 referent 何时被 enqueue 到 ReferenceQueue 中,  但是 WeakReference 也有对应的功能, 两者的区别到底在哪呢 ?这就要说到 Object 的 finalize 方法, 此方法将在 gc 执行前被调用, 如果某个对象重载了 finalize 方法并故意在方法内创建本身的强引用,  这将导致这一轮的 GC 无法回收这个对象并有可能引起任意次 GC， 最后的结果就是明明 JVM 内有很多 Garbage 却 OutOfMemory， 使用 PhantomReference 就可以避免这个问题， 因为 PhantomReference 是在 finalize 方法执行后回收的，也就意味着此时已经不可能拿到原来的引用,  也就不会出现上述问题,  当然这是一个很极端的例子, 一般不会出现. 7. 对比taken from 




Soft vs Weak vs Phantom References

Type
Purpose
Use
When GCed
Implementing Class


Strong Reference
An ordinary reference. Keeps objects alive as long as they are referenced.
normal reference.
Any object not pointed to can be reclaimed.
default

Soft Reference
Keeps objects alive provided there’s enough memory.
to keep objects alive even after clients have removed their references (memory-sensitive caches), in case clients start asking for them again by key.
After a first gc pass, the JVM decides it still needs to reclaim more space.
java.lang.ref.SoftReference 

Weak Reference
Keeps objects alive only while they’re in use (reachable) by clients.
Containers that automatically delete objects no longer in use.
After gc determines the object is only weakly reachable
java.lang.ref.WeakReference java.util.WeakHashMap 

Phantom Reference
Lets you clean up after finalization but before the space is reclaimed (replaces or augments the use offinalize())
Special clean up processing
After finalization.
java.lang.ref.PhantomReference8. 小结       一般的应用程序不会涉及到 Reference 编程， 但是了解这些知识会对理解 GC 的工作原理以及性能调优有一定帮助,   在实现一些基础性设施比如缓存时也可能会用到， 希望本文能有所帮助.        王政 于 2009，6，3                                   
            
            
              
			  
			  
				
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  	//测试字符串的长度 一个汉字算2个字节
	function mb_strlen(str)
	{
		var len=str.length;
		var totalCount=0;
		for(var i=0;i<len;i++)
		{
			var c = str.charCodeAt(i);
			if ((c >= 0x0001 && c <= 0x007e) || (0xff60<=c && c<=0xff9f)) {
				totalCount++;
			}else{
				totalCount+=2;
			}
		}
		return totalCount;
	}
	/*
	var Util = {};
	Util.calWbText = function (text, max){
		if(max === undefined)
			max = 500;
		var cLen=0;
		var matcher = text.match(/[^\x00-\xff]/g),
			wlen  = (matcher && matcher.length) || 0;
		//匹配url链接正则 http://***
		var pattern = /http:\/\/([\w-]+\.)+[\w-]+(\/*[\w-\.\/\?%&=][^\s^\u4e00-\u9fa5]*)?/gi;
		//匹配的数据存入数组
		var arrPt = new Array();
		var i = 0;
		while((result = pattern.exec(text)) != null){
			arrPt[i] = result[0];
			i++;
		}
		//替换掉原文本中的链接
		for(var j = 0;j<arrPt.length;j++){
			text = text.replace(arrPt[j],"");
		}
		//减掉链接所占的长度
		return Math.floor((max*2 - text.length - wlen)/2 - 12*i);
	};
	*/
	var allowComment = '0';
	
	//举报弹出层
	function showJuBao(url, cid){
		
			$.cover(false);
			asyncbox.open({
				id  : 'report_thickbox',
				url : url,
				title : '举报违规',
				width : 378,
				height : 240,
				scroll : 'no',
				data : {
					'cid'	 : cid,
					'idtype' : 2 ,
                                        'blogurl' : window.location.href
				},
				callback : function(action){
					if(action == 'close'){
						$.cover(false);
					}
				}
			});
	}

	$(function(){

		//创建管理员删除的弹出层
		$('#admin_article_del').click(function(){
			$.cover(false);
			asyncbox.open({
				id : 'class_thickbox',
				html : '<div class="HT_layer3_1"><ul><li class="HT_li1">操作原因：<select class="HT_sel7" id="del_type" name="del_type"><option value="广告文章">广告文章</option><option value="违规内容">违规内容</option><option value="标题不明">标题不明</option><option value="文不对题">文不对题</option></select></li><li class="HT_li1" ><input class="HT_btn4" id="admin_delete"  type="button" /></li></ul></div>',
				title : '选择类型',
				width : 300,
				height : 150,
				scroll : 'no',
				callback : function(action){
					if(action == 'close'){
						$.cover(false);
					}
				}
			});
		});
		$('#admin_delete').live('click' , function(){
			///blog/logicdel/id/3480184/url/%252Fblog%252Findex.html.html
			var type = $('#del_type').val();
			var url = '/blog/logicdel/id/361000/url/%252Fuid%252F96923.html.html';
			window.location.href= url + '?type=' + type;
		});


		//顶  js中暂未添加&过滤 
		$('#digg').live('click' , function(){

			if(isOnLine == '' )
			{
				//showErrorMsg('登录之后才能进行此操作' , '消息提示');
				showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
				return false;
			}

			var bid = $('#digg').attr('bid');
			var url = $('#digg').attr('url');

			var digg_str = $.cookie('digg_id');
			if(digg_str != null)
			{
				var arr= new Array(); //定义一数组

				arr = digg_str.split(","); //字符分割     
				for( i=0 ; i < arr.length ; i++ )   
				{   
					if(bid == arr[i])
					{
						showErrorMsg('已经赞过该文章', '消息提示'); 
						return false;
					}
				} 
			}
			$.ajax({
				type:"POST",
				url:url,	
				data: {
					'bid' : bid 
				},
				dataType: 'json',
				success:function(msg){
					if(msg.error == 0)
					{
						var num = parseInt($('#digg_num').html(),10);
						num += 1;
						$('#digg_num').html(num);
						$('#digg').die();

						if(digg_str == null)
						{
							$.cookie('digg_id', bid, {expires: 30 , path: '/'});
						}
						else
						{
							$.cookie('digg_id', digg_str + ',' + bid, {expires: 30 , path: '/'});
						}
						showSucceedMsg('谢谢' , '消息提示');
					}
					else if(msg.error == 1)
					{
						//showErrorMsg(msg.error_content , '消息提示'); 
						showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
					}
					else if(msg.error == 2)
					{
						showErrorMsg(msg.error_content , '消息提示'); 
					}
				}
			});
		});
		//举报弹出层
		/*$('.box_report').live('click' , function(){
			if(isOnLine == '' )
			{
				//showErrorMsg('登录之后才能进行此操作' , '消息提示');
				showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
				return false;
			}
			var url = $('#report_url').val();
			var cid = $(this).attr('cid');
			$.cover(false);
			asyncbox.open({
				id  : 'report_thickbox',
				url : url,
				title : '举报违规',
				width : 378,
				height : 240,
				scroll : 'no',
				data : {
					'cid'	 : cid,
					'idtype' : 2 
				},
				callback : function(action){
					if(action == 'close'){
						$.cover(false);
					}
				}
			});
		});*/


		//评论相关代码
		
		//点击回复显示评论框
		$('.Blog_a10').live('click' , function(){
			if(isOnLine == '' )
			{
				//showErrorMsg('登录之后才能进行此操作' , '消息提示');
				showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
				return false;
			}

			if(allowComment == 1)
			{
				showErrorMsg('该博文不允许评论' , '消息提示'); 
				return false;
			}
			var tid = $(this).attr('toid');//留言作者id
			var bid = $(this).attr('bid');//blogid
			var cid = $(this).attr('cid');//留言id
			var tname = $(this).attr('tname');
                         			var tpl  = '<div class="Blog_right1_9">';
				tpl +=	 '<div class="div2">';
				tpl +=     '<textarea name="" cols="" rows="" class="Blog_ar1_1" id="rmsg">文明上网，理性发言...</textarea>';
				tpl +=	 '</div>';
				tpl +=   '<div class="div3">';
				tpl +=		'<div class="div3_2"><a href="javascript:void(0);" class="Blog_a11" id="quota_sbumit" url="/Comment/PostComment.html" tid="'+tid+'" bid="'+bid+'" cid="'+cid+'" tname="'+tname+'" ></a><a href="javascript:void(0)" id="qx_comment" class="Blog_a12"></a></div>';
				tpl +=		'<div class="div3_1"><a href="javascript:void(0);" id="mface"><span></span>表情</a></div>';
				tpl +=		'<div class="clear"></div>';
				tpl +=	 '</div>';
				tpl +=  '</div>';
                                 			$('.z_move_comment').html('');
			$(this).parents('.Blog_right1_8').find('.z_move_comment').html(tpl).show();
		});
		//引用的评论提交
		$('#quota_sbumit').live('click' , function(){

			if(isOnLine == '' )
			{
				//showErrorMsg('登录之后才能进行此操作' , '消息提示');
				showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
				return false;
			}

			var bid   = $(this).attr('bid');
			var tid   = $(this).attr('tid');//被引用人的id
			var qid   = $(this).attr('cid');//引用的id
			var url = $(this).attr('url');
			var text = $('#rmsg').val();
			var tname = $(this).attr('tname');
			if(text == '' || text=='文明上网，理性发言...')
			{
				showErrorMsg('评论内容不能为空！' , '消息提示'); 
				return false;
			}
			else
			{
				if(mb_strlen(text) > 1000){
					showErrorMsg('评论内容不能超过500个汉字' , '消息提示'); 
					return false;
				}
			}
		    $.ajax({
		        type: "post",
			    url: url ,
			    data: {'bid': bid , 'to' : tid , 'qid' : qid , 'text': text , 'tname' : tname },
				dataType: 'json',
			    success: function(data){
				    if(data.code == 1){
						var tpl =  '<div class="Blog_right1_8">';
							tpl+=     '<div class="Blog_right_img1"><a href="' +data.info.url+ '" >' + data.info.header + '</a></div>';
							tpl+=     '<div class="Blog_right_font1">';
							tpl+=         '<p class="Blog_p5"><span><a href="' +data.info.url+ '" >' + data.info.username + '</a></span>' + data.info.dateline + '</p>';
							tpl+=         '<p class="Blog_p7"><a href="' + data.info.quota.url + '">' + data.info.quota.username + '</a>：'+ data.info.quota.content + '</p>';
							tpl+=         '<p class="Blog_p8">' + data.info.content + '</p><span class="span_text1"><a href="javascript:void(0);" class="Blog_a10" toid=' + data.info.fuid + ' bid=' + data.info.bid + ' cid=' + data.info.cid + '  tname = ' + data.info.username + ' >回复</a> | 　<a class="comment_del_mark" style="cursor:pointer" url="' + data.info.delurl + '" >删除</a>　| 　<a href="javascript:showJuBao(\'/blog/ViewReport.html\','+data.info.cid+')" class="box_report" cid="' + data.info.cid + '" >举报</a></span></div>';
							tpl+=         '<div class="z_move_comment" style="display:none"></div>';
							tpl+=	      '<div class="Blog_line1"></div></div>';
							$('#replyList .Blog_right1_8:first').before(tpl);
							$('.z_move_comment').html('').hide();
				    }
					else if(data.code == -1){
						//showErrorMsg(data.info , '消息提示'); 
						showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
			        }
                },
			    error: function(){//请求出错处理
                        
                }
			});
		});
		//底部发表评论
		$('#submitmsg').click(function(){
			if(allowComment == 1)
			{
				showErrorMsg('该博文不允许评论' , '消息提示'); 
				return false;
			}
			var bid   = $(this).attr('bid');
			var toid  = $(this).attr('toid');
			var text = $('#reply').val();
			var url = $(this).attr('url');
			if(text == '' || text=='文明上网，理性发言...')
			{
				showErrorMsg('评论内容不能为空！' , '消息提示'); 
				return false;
			}
			else
			{
				if(mb_strlen(text) > 1000){
					showErrorMsg('评论内容不能超过500个汉字' , '消息提示'); 
					return false;
				}
			}
		    $.ajax({
		        type: "post",
			    url: url ,
			    data: {'bid': bid , 'to' : toid ,'text': text},
				dataType: 'json',
			    success: function(data){
				    if(data.code == 1)
					{
						var tpl   = '<div class="Blog_right1_8">';
							tpl  +=   '<div class="Blog_right_img1"><a href="' +data.info.url+ '" >' + data.info.header + '</a></div>';
							tpl  +=	  '<div class="Blog_right_font1">';
							tpl  +=       '<p class="Blog_p5"><span><a href="' +data.info.url+ '" >' + data.info.username + '</a></span>' + data.info.dateline + '</p>';
							tpl  +=       '<p class="Blog_p6">' + data.info.content + '</p>';
							tpl  +=		  '<div class="div1"><a href="javascript:void(0);" class="Blog_a10"  toid=' + data.info.fuid + ' bid=' + data.info.bid + ' cid=' + data.info.cid + '>回复</a> | 　<a class="comment_del_mark" style="cursor:pointer" url="' + data.info.delurl + '">删除</a>　| 　<a href="javascript:showJuBao(\'/blog/ViewReport.html\','+data.info.cid+')" class="box_report" cid="' + data.info.cid + '">举报</a></div>';
							tpl  +=		  '<div class="z_move_comment" style="display:none"></div>';
							tpl  +=    '</div><div class="Blog_line1"></div></div>';
							$('.Blog_tit3:first').after(tpl);
							$('#reply').val('文明上网，理性发言...');
					}
					else if(data.code == -1)
					{
						showErrorMsg(data.info , '消息提示'); 
			        }
                },
			    error: function(){//请求出错处理
                        
                }
		    });
			
		});
		//底部评论重置
		$('#reset_comment').click(function(){
			$('#reply').val('文明上网，理性发言...');
		});
		//取消回复
		$('#qx_comment').live('click' , function(){
			$('.z_move_comment').html('').hide();
		});


		$('#rmsg, #reply').live({
		    focus:function(){
			    if($(this).val() == '文明上网，理性发言...'){
			        $(this).val('');
			    }
			},
			blur:function(){
		        if($(this).val() == ''){
			        $(this).val('文明上网，理性发言...');
			    }
			}
		});
		//删除留言确认
		$('.comment_del_mark').live('click' , function(){
			var url = $(this).attr('url');
			asyncbox.confirm('删除留言','确认', function(action){
				if(action == 'ok')
				{
					location.href = url;
				}
			});
		});
		//删除时间确认
		$('.del_article_id').click(function(){
			var delurl = $(this).attr('delurl');
			asyncbox.confirm('删除文章','确认', function(action){
				if(action == 'ok')
				{
					location.href = delurl;
				}
			});
		});
		/*
		//字数限制
		$('#rmsg, #reply').live('keyup', function(){
			var id = $(this).attr('id');
			var left = Util.calWbText($(this).val(), 500);
			var eid = '#errmsg';
			
			if(id == 'reply') eid =  '#rerrmsg';
			if (left >= 0)
		        $(eid).html('您还可以输入<span>' + left + '</span>字');
		    else
		        $(eid).html('<font color="red">您已超出<span>' + Math.abs(left) + '</span>字 </font>');
		});
		*/
		//加载表情
	    $('#face').qqFace({id : 'facebox1', assign : 'reply', path : '/image/qqface/'});
	    $('#mface').qqFace({id : 'facebox', assign : 'rmsg', path:'/image/qqface/'});
	    
		/*
		$('#class_one_id').change(function(){
			alert(123213);
			var id = parseInt($(this).val() , 10);
			if(id == 0) return false;
			$('.hidden_son_class span').each(function( index , dom ){
				if( dom.attr('cid')  == id )
				{
				}
			});
		});
		*/
		//转载文章
		var turn_url = "/blog/viewClassPart.html";
		$('#repost_bar').click(function(){
			if(isOnLine == '' )
			{
				//showErrorMsg('登录之后才能进行此操作' , '消息提示');
				showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
				return false;
			}
			var id = $(this).attr('bid');
			asyncbox.open({
				id  : 'turn_class_thickbox',
				url : turn_url,
				title : '转载文章',
				width : 330,
				height : 131,
				scroll : 'no',
				data : {
					'id'	 : id
				},
				callback : function(action){
					if(action == 'close'){
						$.cover(false);
					}
				}
			});
		});
		/*
	    //转发文章
	    $('#repost_bar').live('click' , function(){
			if(isOnLine == '' )
			{
				//showErrorMsg('登录之后才能进行此操作' , '消息提示');
				showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
				return false;
			}
	    	var bid  = $(this).attr('bid');
	    	var url  = $(this).attr('url');
	    	asyncbox.confirm('转载文章','确认', function(action){
	    	  if(action == 'ok'){
	    	    $.ajax({
				type:"POST",
				url:url,	
				data: {
					'bid' : bid 
				},
				dataType: 'json',
				success:function(msg){
				  if(msg.error == 0){
				    showSucceedMsg('转发成功！', '消息提示');
				  }else if(msg.error == 1){
				    //location.href = '/index.php?r=site/login';
					showErrorMsg('操作失败,您需要先登录!', '消息提示', 'http://account.chinaunix.net/login');
				  }else{
				    showErrorMsg(msg.error_content, '消息提示');
			      }
				}
			  });
	    	  }
	    	});

		});
		*/

	});




SyntaxHighlighter.autoloader(
	'applescript			/highlight/scripts/shBrushAppleScript.js',
	'actionscript3 as3		/highlight/scripts/shBrushAS3.js',
	'bash shell				/highlight/scripts/shBrushBash.js',
	'coldfusion cf			/highlight/scripts/shBrushColdFusion.js',
	'cpp c					/highlight/scripts/shBrushCpp.js',
	'c# c-sharp csharp		/highlight/scripts/shBrushCSharp.js',
	'css					/highlight/scripts/shBrushCss.js',
	'delphi pascal			/highlight/scripts/shBrushDelphi.js',
	'diff patch pas			/highlight/scripts/shBrushDiff.js',
	'erl erlang				/highlight/scripts/shBrushErlang.js',
	'groovy					/highlight/scripts/shBrushGroovy.js',
	'java					/highlight/scripts/shBrushJava.js',
	'jfx javafx				/highlight/scripts/shBrushJavaFX.js',
	'js jscript javascript	/highlight/scripts/shBrushJScript.js',
	'perl pl				/highlight/scripts/shBrushPerl.js',
	'php					/highlight/scripts/shBrushPhp.js',
	'text plain				/highlight/scripts/shBrushPlain.js',
	'py python				/highlight/scripts/shBrushPython.js',
	'ruby rails ror rb		/highlight/scripts/shBrushRuby.js',
	'scala					/highlight/scripts/shBrushScala.js',
	'sql					/highlight/scripts/shBrushSql.js',
	'vb vbnet				/highlight/scripts/shBrushVb.js',
	'xml xhtml xslt html	/highlight/scripts/shBrushXml.js'
);
SyntaxHighlighter.all();


function code_hide(id){
	var code = document.getElementById(id).style.display;
	if(code == 'none'){
		document.getElementById(id).style.display = 'block';
	}else{
		document.getElementById(id).style.display = 'none';
	}
}



 lastScrollY=0;
 function heartBeat(){
 var diffY;
 if (document.documentElement && document.documentElement.scrollTop)
     diffY = document.documentElement.scrollTop;
 else if (document.body)
     diffY = document.body.scrollTop
 else
     {/*Netscape stuff*/}
 percent=.1*(diffY-lastScrollY);
 if(percent>0)percent=Math.ceil(percent);
 else percent=Math.floor(percent);
 document.getElementById("full").style.top=parseInt(document.getElementById("full").style.top)+percent+"px";
 lastScrollY=lastScrollY+percent;
 if(lastScrollY<200)
 {
 document.getElementById("full").style.display="none";
 }
 else
 {
 document.getElementById("full").style.display="block";
 }
 }
 var gkuan=document.body.clientWidth;
 var ks=(gkuan-960)/2-30;
 suspendcode="<div id=\"full\" style='right:-30px;POSITION:absolute;TOP:500px;z-index:100;width:26px; height:86px;cursor:pointer;'><a href=\"javascript:void(0)\" onclick=\"window.scrollTo(0,0);\"><img src=\"\/image\/top.png\" /></a></div>"
 document.write(suspendcode);
 window.setInterval("heartBeat()",1);
 
  
  
    关于我们 | 关于IT168 | 联系方式 | 广告合作 | 法律声明 | 免费注册
      Copyright  2001-2010 ChinaUnix.net All Rights Reserved 北京皓辰网域网络信息技术有限公司. 版权所有 
      感谢所有关心和支持过ChinaUnix的朋友们
        16024965号-6 
      
    
  



//全局错误提示弹出框
function showErrorMsg(content, title, url){
	var url = url || '';
	var title = title || '消息提示';
	var html = '';
	html += '<div class="HT_layer3_1 HT_layer3_2"><ul><li><p><span class="login_span1"></span>' + content + '</p></li>';
	if(url == '' || url.length == 0){
		html += '<li class="HT_li1"><input type="button" class="HT_btn2"  onclick=\'close_windows("error_msg")\'></li>';
	} else {
		html += '<li class="HT_li1"><input type="button" class="login_btn1" onclick="location.href=\'' + url + '\'"></li>';
	}
	html += '</ul></div>';
	$.cover(true);
	   asyncbox.open({
		 id: 'error_msg',
		 title : title,
		 html : html,
		 'callback' : function(action){
			 if(action == 'close'){
				 $.cover(false);
			 }
		 }
	});
}

//全局正确提示
function showSucceedMsg(content, title , url ){
	var url = url || '';
	var title = title || '消息提示';
	var html = '';
	html += '<div class="HT_layer3_1 HT_layer3_2"><ul><li><p><span class="login_span2"></span>' + content + '</p></li>';
	if(url == '' || url.length == 0){
		html += '<li class="HT_li1"><input type="button" class="HT_btn2"  onclick=\'close_windows("error_msg")\'></li>';
	} else {
		html += '<li class="HT_li1"><input type="button" class="HT_btn2" onclick="location.href=\'' + url + '\'"></li>';
	}
	html += '</ul></div>';
	$.cover(true);
	asyncbox.open({
		 id: 'error_msg',
		 title : title,
		 html : html,
		 'callback' : function(action){
			 if(action == 'close'){
				 $.cover(false);
			 }
		 }
	});
}

//关闭指定id的窗口
function close_windows(id){
	$.cover(false);
	$.close(id);
}


//公告
var tID;
var tn;                        // 高度
var nStopTime = 5000;        // 不同行间滚动时间隔的时间，值越小，移动越快
var nSpeed = 50;            // 滚动时，向上移动一像素间隔的时间，值越小，移动越快
var isMove = true;
var nHeight = 25;
var nS = 0;
var nNewsCount = 3;

/**
 * n 用于表示是否为第一次运行
 **/
function moveT(n)
{
    clearTimeout(tID)
    var noticev2 = document.getElementById("noticev2")
    nS = nSpeed;

    // 只在第一次调用时运行，初始化环境（有没有参数）
    if (n)
    {
        // 设置行高
        noticev2.style.lineHeight = nHeight + "px";
        // 初始化显示位置
        tn = 0;
        // 刚进入时在第一行停止时间
        nS = nStopTime;
    }

    // 判断鼠标是否指向层
    if (isMove)
    {
        // 向上移动一像素
        tn--;
        // 如果移动到最下面一行了，则移到顶行
        if (Math.abs(tn) == nNewsCount * nHeight)
        {
            tn = 0;
        }
        // 设置位置
        noticev2.style.marginTop = tn + "px";
        // 完整显示一行时，停止一段时间
        if (tn % nHeight == 0)
        {
            nS = nStopTime;
        }
    }

    tID = setTimeout("moveT()", nS);
}
moveT(1);    // 此处可以传入任何参数


//  var _gaq = _gaq || [];
//  _gaq.push(['_setAccount', 'UA-20237423-2']);
//  _gaq.push(['_setDomainName', '.chinaunix.net']);
//  _gaq.push(['_trackPageview']);
//
//  (function() {
//    var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
//    ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
//    var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
//  })();



var _bdhmProtocol = (("https:" == document.location.protocol) ? " https://" : " http://");
document.write(unescape("%3Cscript src='" + _bdhmProtocol + "hm.baidu.com/h.js%3F0ee5e8cdc4d43389b3d1bfd76e83216b' type='text/javascript'%3E%3C/script%3E"));

function link(t){
     var href= $(t).attr('href');
     href+="?url="+encodeURIComponent(location.href);
     $(t).attr('href',href);
    //setCookie("returnOutUrl", location.href, 60, "/");
}

    assertFalse(weakReference.isEnqueued());
    Referenceextends Object> polled = referenceQueue.poll();
    assertNull(polled);

    referent = null;
    System.gc();

    assertTrue(weakReference.isEnqueued());
    Referenceextends Object> removed = referenceQueue.remove();
    assertNotNull(removed);
}

    /**
     * phantom reference 的 get 方法永远返回 null
     */
    assertNull(phantomReference.get());
}

    assertNotNull(softRerference.get());

    referent = null;
    System.gc();

    /**
     *  soft references 只有在 jvm OutOfMemory 之前才会被回收, 所以它非常适合缓存应用
     */
    assertNotNull(softRerference.get());
}

    assertSame(referent, weakRerference.get());

    referent = null;
    System.gc();

    /**
     * 一旦没有指向 referent 的强引用, weak reference 在 GC 后会被自动回收
     */
    assertNull(weakRerference.get());
}