在 Java 中，有四種類型的引用在它們被垃圾收集的方式上有所不同。

1.強(qiáng)引用

2.弱引用

3.軟引用

4.幻影參考

強(qiáng)引用：

這是引用對(duì)象的默認(rèn)類型/類。任何具有活動(dòng)強(qiáng)引用的對(duì)象都沒(méi)有資格進(jìn)行垃圾回收。只有當(dāng)被強(qiáng)引用的變量指向 null 時(shí)，對(duì)象才會(huì)被垃圾回收。

MyClass obj = new MyClass();          

這里的 'obj' 對(duì)象是對(duì)新創(chuàng)建的 MyClass 實(shí)例的強(qiáng)引用，當(dāng)前 obj 是活動(dòng)對(duì)象，因此不能被垃圾收集。

對(duì)象=空；
//'obj' 對(duì)象不再引用實(shí)例。
所以'MyClass 類型對(duì)象現(xiàn)在可用于垃圾收集。

// Java program to illustrate Strong reference
class Gfg
{
	//Code..
}
public class Example
{
	public static void main(String[] args)
	{
		//Strong Reference - by default
		Gfg g = new Gfg();			
		//Now, object to which 'g' was pointing earlier is
		//eligible for garbage collection.
		g = null;
	}
}

弱引用：

弱引用對(duì)象不是引用對(duì)象的默認(rèn)類型/類，在使用它們時(shí)應(yīng)明確指定。

這種類型的引用在 WeakHashMap 中用于引用條目對(duì)象。

如果 JVM 檢測(cè)到一個(gè)只有弱引用的對(duì)象(即沒(méi)有強(qiáng)或軟引用鏈接到任何對(duì)象對(duì)象)，該對(duì)象將被標(biāo)記為垃圾回收。

要?jiǎng)?chuàng)建此類引用，使用java.lang.ref.WeakReference類。

這些引用在實(shí)時(shí)應(yīng)用程序中使用，同時(shí)建立一個(gè) DBConnection，當(dāng)使用數(shù)據(jù)庫(kù)的應(yīng)用程序關(guān)閉時(shí)，垃圾收集器可能會(huì)清理該 DBConnection。

//Java Code to illustrate Weak reference
import java.lang.ref.WeakReference;
class Gfg
{
	//code
	public void x()
	{
		System.out.println("GeeksforGeeks");
	}
}
public class Example
{
	public static void main(String[] args)
	{
		// Strong Reference
		Gfg g = new Gfg();	
		g.x();		
		// Creating Weak Reference to Gfg-type object to which 'g'
		// is also pointing.
		WeakReference<Gfg> weakref = new WeakReference<Gfg>(g);		
		//Now, Gfg-type object to which 'g' was pointing earlier
		//is available for garbage collection.
		//But, it will be garbage collected only when JVM needs memory.
		g = null;		
		// You can retrieve back the object which
		// has been weakly referenced.
		// It successfully calls the method.
		g = weakref.get();		
		g.x();
	}
}

輸出：

GeeksforGeeks
GeeksforGeeks

可以招募兩種不同級(jí)別的弱點(diǎn)：軟弱點(diǎn)和幻影弱點(diǎn)

軟引用：

在軟引用中，即使對(duì)象可以進(jìn)行垃圾回收，也不會(huì)被垃圾回收，直到 JVM 嚴(yán)重需要內(nèi)存。當(dāng) JVM 內(nèi)存不足時(shí)，對(duì)象會(huì)從內(nèi)存中清除。創(chuàng)建這樣的引用使用java.lang.ref.SoftReference類。

//Code to illustrate Soft reference
import java.lang.ref.SoftReference;
class Gfg
{
	//code..
	public void x()
	{
		System.out.println("GeeksforGeeks");
	}
}
public class Example
{
	public static void main(String[] args)
	{
		// Strong Reference
		Gfg g = new Gfg();	
		g.x();		
		// Creating Soft Reference to Gfg-type object to which 'g'
		// is also pointing.
		SoftReference<Gfg> softref = new SoftReference<Gfg>(g);		
		// Now, Gfg-type object to which 'g' was pointing
		// earlier is available for garbage collection.
		g = null;		
		// You can retrieve back the object which
		// has been weakly referenced.
		// It successfully calls the method.
		g = softref.get();		
		g.x();
	}
}

輸出：

GeeksforGeeks
GeeksforGeeks

幻影引用：

幻影引用所引用的對(duì)象符合垃圾回收條件。但是，在將它們從內(nèi)存中刪除之前，JVM 會(huì)將它們放入一個(gè)名為 'reference queue' 的隊(duì)列中。在對(duì)它們調(diào)用 finalize() 方法后將它們放入引用隊(duì)列中。使用java.lang.ref.PhantomReference類創(chuàng)建此類引用。

//Code to illustrate Phantom reference
import java.lang.ref.*;
class Gfg
{
	//code
	public void x()
	{
		System.out.println("GeeksforGeeks");
	}
}
public class Example
{
	public static void main(String[] args)
	{
		//Strong Reference
		Gfg g = new Gfg();	
		g.x();		
		//Creating reference queue
		ReferenceQueue<Gfg> refQueue = new ReferenceQueue<Gfg>();
		//Creating Phantom Reference to Gfg-type object to which 'g'
		//is also pointing.
		PhantomReference<Gfg> phantomRef = null;
		phantomRef = new PhantomReference<Gfg>(g,refQueue);		
		//Now, Gfg-type object to which 'g' was pointing
		//earlier is available for garbage collection.
		//But, this object is kept in 'refQueue' before
		//removing it from the memory.
		g = null;		
		//It always returns null.
		g = phantomRef.get();		
		//It shows NullPointerException.
		g.x();
	}
}

運(yùn)行時(shí)錯(cuò)誤：

線程“主”java.lang.NullPointerException 中的異常
    在 Example.main(Example.java:31)

輸出：

GeeksforGeeks