什么时候以及如何在 Java 中垃圾收集类? [英] When and how are classes garbage collected in Java?
问题描述
我在本主题.但是我得到的答案又给了我另一个问题.
有人提到垃圾收集器也可以收集类.这是真的?
如果是真的,这是如何工作的?
Java 中的类可以在没有任何引用时被垃圾收集.在大多数简单的设置中,这种情况永远不会发生,但在某些情况下可能会发生.
有很多方法可以使类可访问,从而阻止其符合 GC 的条件:
- 该类的对象仍然可以访问.
- 代表该类的
Class
对象仍然可以访问 - 加载类的
ClassLoader
仍然可以访问 ClassLoader
加载的其他类仍然可以访问
当 none 为真时,ClassLoader
和它加载的所有类都有资格进行 GC.
这是一个构建的示例(充满了不好的做法!),它应该展示行为:
在目录(不是包!)x
中创建一个字节码文件GCTester.class
.它的源代码是:
公共类 GCTester {public static final GCTester INSTANCE=new GCTester();私人 GCTester() {System.out.println(this + "created");}公共无效终结(){System.out.println(this + "finalized");}}
然后在x
的父目录下创建一个类TestMe
:
import java.io.File;导入 java.net.URL;导入 java.net.URLClassLoader;导入 java.lang.reflect.Field;公共类 TestMe {public static void main(String[] args) 抛出异常 {System.out.println("in main");testGetObject();System.out.println("第二次 gc() 调用(在 main 中)");System.gc();线程睡眠(1000);System.out.println("主程序结束");}公共静态无效 testGetObject() 抛出异常 {System.out.println("创建类加载器");ClassLoader cl = new URLClassLoader(new URL[] {new File("./x").toURI().toURL()});System.out.println("加载类");类>clazz = cl.loadClass("GCTester");System.out.println("获取静态字段");Field field = clazz.getField("INSTANCE");System.out.println("读取静态值");Object object = field.get(null);System.out.println("得到值:" + object);System.out.println("第一次 gc() 调用");System.gc();线程睡眠(1000);}}
运行 TestMe
将产生这个(或类似的)输出:
在倒数第二行中,我们看到 GCTester
实例已完成,这只能表示该类(和 ClassLoader
)有资格进行垃圾回收.
I asked a question about Garbage Collection in Java in this topic. But the answer I got, gave me another question.
Someone mentioned that classes can be collected by the garbage collector too. Is this true?
And if it is true, how does this work?
A class in Java can be garbage-collected when nothing references it. In most simple setups this never happens, but there are situations where it can occur.
There are many ways to make a class reachable and thus prevent it from being eligible for GC:
- objects of that class are still reachable.
- the
Class
object representing the class is still reachable - the
ClassLoader
that loaded the class is still reachable - other classes loaded by the
ClassLoader
are still reachable
When none of those are true, then the ClassLoader
and all classes it loaded are eligible for GC.
Here's a constructed example (full of bad practices!) that should demonstrate the behaviour:
Create a bytecode file GCTester.class
in a directory (not package!) x
. It's source code is:
public class GCTester {
public static final GCTester INSTANCE=new GCTester();
private GCTester() {
System.out.println(this + " created");
}
public void finalize() {
System.out.println(this + " finalized");
}
}
Then create a class TestMe
in the parent directory of x
:
import java.io.File;
import java.net.URL;
import java.net.URLClassLoader;
import java.lang.reflect.Field;
public class TestMe {
public static void main(String[] args) throws Exception {
System.out.println("in main");
testGetObject();
System.out.println("Second gc() call (in main)");
System.gc();
Thread.sleep(1000);
System.out.println("End of main");
}
public static void testGetObject() throws Exception {
System.out.println("Creating ClassLoader");
ClassLoader cl = new URLClassLoader(new URL[] {new File("./x").toURI().toURL()});
System.out.println("Loading Class");
Class<?> clazz = cl.loadClass("GCTester");
System.out.println("Getting static field");
Field field = clazz.getField("INSTANCE");
System.out.println("Reading static value");
Object object = field.get(null);
System.out.println("Got value: " + object);
System.out.println("First gc() call");
System.gc();
Thread.sleep(1000);
}
}
Running TestMe
will produce this (or similar) output:
in main Creating ClassLoader Loading Class Getting static field Reading static value GCTester@1feed786 created Got value: GCTester@1feed786 First gc() call Second gc() call (in main) GCTester@1feed786 finalized End of main
In the second to last line we see that the GCTester
instance is finalized, which can only mean that the class (and ClassLoader
) are eligible for garbage collection.
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