JVM在调用NETunnelProviderManager.loadAllFromPreferencesWithCompletionHandler时崩溃

Question

我想从我的java应用程序中调用NetworkExtension框架函数。作为第一步，我打电话给NETunnelProviderManager.loadAllFromPreferencesWithCompletionHandler。我使用的是从源代码编译的罗可可图书馆 (0.8.3-快照)。

我的NETunnelProviderManager类如下：

package ai.safekids.infra.jna.ext.mac.networkextension;

import ai.safekids.infra.jna.ext.mac.data.NSArrayRef;
import ai.safekids.infra.jna.ext.mac.data.NSErrorRef;
import com.sun.jna.Callback;
import org.rococoa.Rococoa;
import org.rococoa.ObjCClass;
import org.rococoa.cocoa.foundation.NSArray;

public abstract class NETunnelProviderManager extends NEVPNManager {
    public static final _Class CLASS = Rococoa.createClass("NETunnelProviderManager", _Class.class);

    public interface LoadAllFromPreferencesCompletionHandler extends Callback {
        void callback(NSArrayRef managers, NSErrorRef error);
    }

    public static NETunnelProviderManager new_() {
        return Rococoa.create("NETunnelProviderManager", NETunnelProviderManager.class);
    }

    public interface _Class extends ObjCClass {
        void loadAllFromPreferencesWithCompletionHandler(LoadAllFromPreferencesCompletionHandler completionHandler);
    }

    public static void loadAllFromPreferencesWithCompletionHandler(LoadAllFromPreferencesCompletionHandler completionHandler) {
        CLASS.loadAllFromPreferencesWithCompletionHandler(completionHandler);
    }
}

而NEVPNManager类是：

package ai.safekids.infra.jna.ext.mac.networkextension;

import com.sun.jna.Callback;
import org.rococoa.Rococoa;
import org.rococoa.cocoa.foundation.NSError;
import org.rococoa.cocoa.foundation.NSObject;
import org.rococoa.ObjCClass;

public abstract class NEVPNManager extends NSObject {
    public static final _Class CLASS = Rococoa.createClass("NEVPNManager", _Class.class);

    public interface LoadFromPreferencesCompletionHandler extends Callback {
        void invoke(NSError error);
    }

    public interface _Class extends ObjCClass {
        NEVPNManager sharedManager();
    }

    public static NEVPNManager new_() {
        return Rococoa.create("NEVPNManager", NEVPNManager.class);
    }

    static public final NEVPNManager sharedManager = NEVPNManager.CLASS.sharedManager();

    static public NEVPNManager sharedManager() {
        return sharedManager;
    }

    public abstract void loadFromPreferencesWithCompletionHandler(LoadFromPreferencesCompletionHandler completionHandler);
}

使VM崩溃的单元测试是：

public void testLoadAllPreferences() throws Exception {

    NETunnelProviderManager.LoadAllFromPreferencesCompletionHandler completionHandler = new NETunnelProviderManager.LoadAllFromPreferencesCompletionHandler() {
        public void callback(NSArrayRef managers, NSErrorRef error) {
            Assert.assertEquals(true, true);
        }
    };

    NETunnelProviderManager.loadAllFromPreferencesWithCompletionHandler(completionHandler);
    TimeUnit.SECONDS.sleep(5);
}

我的NSArrayRef课程是

package ai.safekids.infra.jna.ext.mac.data;

import org.rococoa.ObjCObjectByReference;

public class NSArrayRef extends ObjCObjectByReference {
}

mvn test输出是

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running ai.safekids.infra.jna.ext.mac.NETunnelProviderManagerTest
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by net.sf.cglib.core.ReflectUtils$1 (file:/Users/mac/.m2/repository/cglib/cglib/3.3.0/cglib-3.3.0.jar) to method java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int,java.security.ProtectionDomain)
WARNING: Please consider reporting this to the maintainers of net.sf.cglib.core.ReflectUtils$1
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007fff2049ebc3, pid=12628, tid=5891
#
# JRE version: OpenJDK Runtime Environment (15.0.2+7) (build 15.0.2+7)
# Java VM: OpenJDK 64-Bit Server VM (15.0.2+7, mixed mode, sharing, tiered, compressed oops, g1 gc, bsd-amd64)
# Problematic frame:
# C  [libobjc.A.dylib+0x5bc3]  objc_retain+0x23
#
# No core dump will be written. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
#
# An error report file with more information is saved as:
# /Users/mac/code/safekids/packet-proxy/hs_err_pid12628.log
#
# If you would like to submit a bug report, please visit:
#   https://bugreport.java.com/bugreport/crash.jsp
# The crash happened outside the Java Virtual Machine in native code.
# See problematic frame for where to report the bug.
#
/bin/sh: line 1: 12628 Abort trap: 6           /usr/local/Cellar/openjdk/15.0.2/libexec/openjdk.jdk/Contents/Home/bin/java -jar /Users/mac/code/safekids/packet-proxy/target/surefire/surefirebooter783506685843365516.jar /Users/mac/code/safekids/packet-proxy/target/surefire/surefire517337276897757629tmp /Users/mac/code/safekids/packet-proxy/target/surefire/surefire_011227232066576961507tmp

Results :

Tests run: 0, Failures: 0, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  2.094 s
[INFO] Finished at: 2021-05-25T16:31:12+05:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:2.12.4:test (default-test) on project packet-proxy: Execution default-test of goal org.apache.maven.plugins:maven-surefire-plugin:2.12.4:test failed: The forked VM terminated without saying properly goodbye. VM crash or System.exit called ? -> [Help 1]

几个问题：

• Rococoa支持回调吗？
• VM崩溃的原因是什么，我如何修复它？
• 是在LoadAllFromPreferencesCompletionHandler中使用NSError/NSArray还是在NSErrorRef/NSArrayRef中使用

Answer 1

Rococoa主要是底层JNA库的一个面向对象的接口，它确实支持回调。

JVM崩溃的可能原因是您的回调超出了可访问范围。一旦在Java中执行了这一行，JVM编译器就认为您已经完成了completionHandler变量，并且它有资格进行垃圾收集。

NETunnelProviderManager.loadAllFromPreferencesWithCompletionHandler(completionHandler);

如果您使用的是JDK 9+，这是最容易用可达栅栏解决的。只需在回调后添加此行，以确保保留强引用：

Reference.reachabilityFence(completionHandler);

如果您正在使用JDK8或更早的版本，最好的解决方法是使用.最后阻止并对最后块中的对象执行一些操作，例如，

try {
    NETunnelProviderManager.loadAllFromPreferencesWithCompletionHandler(completionHandler);
    TimeUnit.SECONDS.sleep(5);
} finally {
    // do something with completionHandler
}

“做某事”可以是一个微不足道的执行(例如，一个toString())，您甚至不需要输出它，但是它将确保Java保持在对象上。

我在OpenJDK 8 (Hotspot)中读过的一种可能的“做某事”方法是在其他地方定义另一种方法，它的工作方式类似于JDK9 9+的可达性围栏。我还没有验证这一点；冒着自己的风险尝试它。这可能不适用于其他JVM。

static void reachabilityFence(Object obj) {
    // do nothing
}

另一种可能的解决方法是关闭回调，将其创建放在一个具有资源的试用块中，该块将保留引用，直到块完成并调用close() (可以是一个非op方法)。