java 中的 java.lang.ref.PhantomReference 类
原文:https://www . geesforgeks . org/Java-lang-ref-phantom reference-class-in-Java/
当我们用 Java 创建一个对象时，默认情况下对象是强的。要创建幻影引用对象，我们必须向 JVM 明确指定它。创建虚拟引用对象是因为虚拟引用对象可以进行垃圾收集，但不能立即收集。相反，它被推入一个引用队列中，这样所有这样排队的引用都可以被清除。
该类的构造函数如下表所示
| Constructor parameter | Constructor description | | --- | --- | | 幻影引用(T，参考队列 q): | Create a new phantom reference, which refers to the given object and is registered in the given queue. It is possible to create an empty queue phantom reference, but such a reference is completely useless: its get method will always return null, and since it has no queue, it will never be queued. |从引用类继承的方法如下:
| 方法名 | 方法描述 | | --- | --- | | get() | 返回此引用对象的引用。因为幻影引用的引用总是不可访问的，所以此方法总是返回 null。 | | 清除() | 清除此引用对象。调用此方法不会导致此对象入队。 | | enque() | 将此引用对象添加到注册它的队列中(如果有)。 | | isEnqueued() | 告知此引用对象是否已由程序或垃圾收集器排入队列。 |例 1:
Java 语言(一种计算机语言，尤用于创建网站)
// Java Program to illustrate PhantomReference class
// of java.lang.ref package
// Importing PhantomReference and ReferenceQueue classes
// from java.lanf.ref package
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
// Class 1
// Helper class
class HelperClass {
// Method inside HelperClass
void Display()
{
// Print statement whenever the function is called
System.out.println("Display Function invoked ...");
}
}
// Class 2
// Main class
public class GFG {
// MyClass
// Main driver method
public static void main(String[] args)
{
// Creating a strong object of HelperClass
HelperClass obj = new HelperClass();
// Creating a reference queue of HelperClass type
ReferenceQueue<HelperClass> rq
= new ReferenceQueue<>();
// Creating a phantom reference object using rq
PhantomReference<HelperClass> pobj
= new PhantomReference<>(obj, rq);
// Display message only
System.out.println(
"-> Calling Display Function using strong object:");
// Calling the display method over the object of
// HelperClass
obj.Display();
// Display message for better readability
System.out.println("-> Object set to null");
obj = null;
// Getting elements in PhantomReference object
// using standard get() method
obj = pobj.get();
// Display status of objects after fetching
// PhantomReference class objects
System.out.println(
"-> Object status after fetching from PhantomReference now : "
+ obj);
// Display message only
System.out.println(
"-> Calling Display Function after retrieving from weak Object");
// Try block to check for exceptions
try {
obj.Display();
}
// Catch block to handle the exceptions
catch (Exception E) {
// Print message when an exception occurred
System.out.println("-> Error : " + E);
}
}
}
Output
-> Calling Display Function using strong object:
Display Function invoked ...
-> Object set to null
-> Object status after fetching from PhantomReference now : null
-> Calling Display Function after retrieving from weak Object
-> Error : java.lang.NullPointerException
因此，可以看出，与软引用和弱引用不同，幻影引用总是返回空值。
例 2:
Java 语言(一种计算机语言，尤用于创建网站)
// Java Program to illustrate PhantomReference class
// of java.lang.ref package
// Importing Phantomreference and RefereenceQueue classes
// from java.lang.ref package
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
// Class 1
// HelperClass
class X {
// Method
// To print simply
void show()
{
// Display message whenever
// show() method is called
System.out.println("show () from X invoked..");
}
}
// Class 2
// Main class
public class GFG {
// Main driver method
public static void main(String[] args)
{
// Creating default object of X class
X obj = new X();
// Creating new reference queue object
ReferenceQueue<X> rq = new ReferenceQueue<X>();
// Creating an object of PhantomReference class
// of X class type with RefereneQueue object
// reference queue
PhantomReference<X> phantomobj
= new PhantomReference<X>(obj, rq);
// Display message
System.out.println(
"-> Trying to retrieve object from Phantom Reference :");
// Try block to check for exceptions
try {
// this will always throw error as it has been
// collected
// by the garbage collector
phantomobj.get().show();
}
// Catch block to handle the exceptions
catch (Exception e) {
// Print and display the exception
System.out.println(e);
}
}
}
Output
-> Trying to retrieve object from Phantom Reference :
java.lang.NullPointerException
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