1.强引用
这是使用最普遍的引用。如果一个对象具有强引用,那就类似于必不可少的生活用品,垃圾回收器绝不会回收它。当内存空 间不足,Java虚拟机宁愿抛出OutOfMemoryError错误,使程序异常终止,也不会靠随意回收具有强引用的对象来解决内存不足问题。
2.软引用(SoftReference)
如果一个对象只具有软引用,那就类似于可有可物的生活用品。如果内存空间足够,垃圾回收器就不会回收它,如果内存空间不足了,就会回收这些对象的内存。只要垃圾回收器没有回收它,该对象就可以被程序使用。软引用可用来实现内存敏感的高速缓存。
软引用可以和一个引用队列(ReferenceQueue)联合使用,如果软引用所引用的对象被垃圾回收,Java虚拟机就会把这个软引用加入到与之关联的引用队列中。
3.弱引用(WeakReference)
如果一个对象只具有弱引用,那就类似于可有可物的生活用品。弱引用与软引用的区别在于:只具有弱引用的对象拥有更短暂的生命周期。在垃圾回收器线程扫描它 所管辖的内存区域的过程中,一旦发现了只具有弱引用的对象,不管当前内存空间足够与否,都会回收它的内存。不过,由于垃圾回收器是一个优先级很低的线程, 因此不一定会很快发现那些只具有弱引用的对象。
弱引用可以和一个引用队列(ReferenceQueue)联合使用,如果弱引用所引用的对象被垃圾回收,Java虚拟机就会把这个弱引用加入到与之关联的引用队列中。
4.虚引用(PhantomReference)
"虚引用"顾名思义,就是形同虚设,与其他几种引用都不同,虚引用并不会决定对象的生命周期。如果一个对象仅持有虚引用,那么它就和没有任何引用一样,在任何时候都可能被垃圾回收。
虚引用主要用来跟踪对象被垃圾回收的活动。虚引用与软引用和弱引用的一个区别在于:虚引用必须和引用队列(ReferenceQueue)联合使用。当垃 圾回收器准备回收一个对象时,如果发现它还有虚引用,就会在回收对象的内存之前,把这个虚引用加入到与之关联的引用队列中。
1 package com.test.reference.mode;
2
3 public class Student {
4
5 private int id;
6 private String name;
7 private int age;
8
9 public Student(int id, String name, int age) {
10 super();
11 this.id = id;
12 this.name = name;
13 this.age = age;
14 }
15
16 @Override
17 protected void finalize() throws Throwable {
18 super.finalize();
19 System.out.println("finalize : " + toString());
20 }
21
22 @Override
23 public String toString() {
24 return " [Student msg : id = " + id + " name = " + name + " age = " + age +"]";
25 }
26
27 public int getId() {
28 return id;
29 }
30 public void setId(int id) {
31 this.id = id;
32 }
33 public String getName() {
34 return name;
35 }
36 public void setName(String name) {
37 this.name = name;
38 }
39 public int getAge() {
40 return age;
41 }
42 public void setAge(int age) {
43 this.age = age;
44 }
45
46 }
3 public class Student {
4
5 private int id;
6 private String name;
7 private int age;
8
9 public Student(int id, String name, int age) {
10 super();
11 this.id = id;
12 this.name = name;
13 this.age = age;
14 }
15
16 @Override
17 protected void finalize() throws Throwable {
18 super.finalize();
19 System.out.println("finalize : " + toString());
20 }
21
22 @Override
23 public String toString() {
24 return " [Student msg : id = " + id + " name = " + name + " age = " + age +"]";
25 }
26
27 public int getId() {
28 return id;
29 }
30 public void setId(int id) {
31 this.id = id;
32 }
33 public String getName() {
34 return name;
35 }
36 public void setName(String name) {
37 this.name = name;
38 }
39 public int getAge() {
40 return age;
41 }
42 public void setAge(int age) {
43 this.age = age;
44 }
45
46 }
1 package com.test.reference;
2
3 import java.lang.ref.ReferenceQueue;
4 import java.lang.ref.WeakReference;
5
6 import com.test.reference.mode.Student;
7
8
9
10 public class ReferenceUsage {
11 public static void main(String args[]) {
12 hold();
13 release();
14 }
15
16 public static void hold() {
17 System.out.println("--------------hold--------------");
18 // Create an object
19 Student stu = new Student(0, "kobe", 33);
20 // Create a reference queue
21 ReferenceQueue<Student> rq = new ReferenceQueue<Student>();
22 // Create a weakReference to obj and associate our reference queue
23 WeakReference<Student> wr = new WeakReference<Student>(stu, rq);
24 System.out.println("weak reference wr: " + wr); // wr
25 System.out.println("weak reference stu wr.get(): " + wr.get()); // stu
26 // Check to see if it´s on the ref queue yet
27 System.out.println("Polling rq.poll(): " + rq.poll()); // null
28 System.gc();
29 System.out.println("hold gc ");
30 System.out.println("Polling rq.poll(): " + rq.poll()); // null
31 System.out.println("weak reference returns obj wr.get(): " + wr.get()); // not null
32 }
33
34 public static void release() {
35 System.out.println("--------------release--------------");
36 // Create an object
37 Student stu = new Student(0, "刘德华", 48);
38 // Create a reference queue
39 ReferenceQueue<Student> rq = new ReferenceQueue<Student>();
40 // Create a weakReference to obj and associate our reference queue
41 WeakReference<Student> wr = new WeakReference<Student>(stu, rq);
42 System.out.println("weak reference wr: " + wr); // wr
43 System.out.println("weak reference stu wr.get(): " + wr.get()); // stu
44 // Check to see if it´s on the ref queue yet
45 System.out.println("Polling rq.poll(): " + rq.poll()); // null
46 stu = null;
47 System.gc();
48 System.out.println("release gc ");
49 System.out.println("Polling rq.poll() : " + rq.poll()); //maybe not null
50 System.out.println("weak reference returns obj : wr.get() " + wr.get()); // null
51 }
2
3 import java.lang.ref.ReferenceQueue;
4 import java.lang.ref.WeakReference;
5
6 import com.test.reference.mode.Student;
7
8
9
10 public class ReferenceUsage {
11 public static void main(String args[]) {
12 hold();
13 release();
14 }
15
16 public static void hold() {
17 System.out.println("--------------hold--------------");
18 // Create an object
19 Student stu = new Student(0, "kobe", 33);
20 // Create a reference queue
21 ReferenceQueue<Student> rq = new ReferenceQueue<Student>();
22 // Create a weakReference to obj and associate our reference queue
23 WeakReference<Student> wr = new WeakReference<Student>(stu, rq);
24 System.out.println("weak reference wr: " + wr); // wr
25 System.out.println("weak reference stu wr.get(): " + wr.get()); // stu
26 // Check to see if it´s on the ref queue yet
27 System.out.println("Polling rq.poll(): " + rq.poll()); // null
28 System.gc();
29 System.out.println("hold gc ");
30 System.out.println("Polling rq.poll(): " + rq.poll()); // null
31 System.out.println("weak reference returns obj wr.get(): " + wr.get()); // not null
32 }
33
34 public static void release() {
35 System.out.println("--------------release--------------");
36 // Create an object
37 Student stu = new Student(0, "刘德华", 48);
38 // Create a reference queue
39 ReferenceQueue<Student> rq = new ReferenceQueue<Student>();
40 // Create a weakReference to obj and associate our reference queue
41 WeakReference<Student> wr = new WeakReference<Student>(stu, rq);
42 System.out.println("weak reference wr: " + wr); // wr
43 System.out.println("weak reference stu wr.get(): " + wr.get()); // stu
44 // Check to see if it´s on the ref queue yet
45 System.out.println("Polling rq.poll(): " + rq.poll()); // null
46 stu = null;
47 System.gc();
48 System.out.println("release gc ");
49 System.out.println("Polling rq.poll() : " + rq.poll()); //maybe not null
50 System.out.println("weak reference returns obj : wr.get() " + wr.get()); // null
51 }
52 }
运行结果:
--------------hold--------------
weak reference wr: java.lang.ref.WeakReference@61de33
weak reference stu wr.get(): [Student msg : id = 0 name = kobe age = 33]
Polling rq.poll(): null
hold gc
Polling rq.poll(): null
weak reference returns obj wr.get(): [Student msg : id = 0 name = kobe age = 33]
--------------release--------------
weak reference wr: java.lang.ref.WeakReference@ca0b6
weak reference stu wr.get(): [Student msg : id = 0 name = 刘德华 age = 48]
Polling rq.poll(): null
finalize : [Student msg : id = 0 name = 刘德华 age = 48]
finalize : [Student msg : id = 0 name = kobe age = 33]
release gc
Polling rq.poll() : java.lang.ref.WeakReference@ca0b6
weak reference returns obj : wr.get() null