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Java中有没有一种简单的方法来获取特定类的所有对象实例

发布于 2024-08-15 12:47:13 字数 342 浏览 4 评论 0原文

目前我正在开发一个 Java 代理来收集内存统计信息。借助仪器 API 我可以掌握这些类（并操纵它们）。使用普通 Java，我可以估算每个对象使用的资源。到目前为止，一切都很好。

我现在面临的问题是“如何获取特定类的每个对象实例”。我可以进行字节码操作来获取对象实例，但我希望还有另一个我不知道的 API，可以帮助我实现我的目标，而无需执行如此繁重的侵入性步骤。最后，性能影响应保持在最低限度。有什么想法吗？

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柠栀 2024-08-22 12:47:13

Eclipse 中的调试器可以向您展示一个类的所有实例，因此我查看了 Eclipse 的源代码。 Eclipse 使用 Java 调试有线协议，它允许您（从 Java 6 开始）查找所请求类的所有实例。如果您想走这条路，请获取 Eclipse 源代码的副本并查看 org.eclipse.jdi.internal.ReferenceTypeImpl 的 instances 方法。

更简单的方法是使用 Java 调试接口< /a>.请注意 ReferenceType.instances 方法。

我仍然没有弄清楚如何使用 JDI 连接到正在运行的进程以及如何获取 ReferenceType 的实例。 JDK 包含几个示例，所以我'我确信这是可行的。

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吾家有女初长成 2024-08-22 12:47:13

正如其他答案中所解释的，您可以使用 JDI 协议来做到这一点。在调试模式下运行 JVM

这很简单：您需要使用--agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=56855

，然后就可以连接到远程（或本地 JVM）并列出指定类的所有实例。此外，您不能直接将远程对象转换为真实对象，但您可以访问远程对象的所有字段，甚至是 cal 方法。

这里如何连接到远程 JVM 并获取虚拟机。

    private static VirtualMachine attach(String hostname, String port) throws IOException, IllegalConnectorArgumentsException {
        //getSocketAttaching connector to connect to other JVM using Socket
        AttachingConnector connector = Bootstrap.virtualMachineManager().attachingConnectors()
                .stream().filter(p -> p.transport().name().contains("socket"))
                .findFirst().get();

        //set the arguments for the connector
        Map<String, Argument> arg = connector.defaultArguments();
        arg.get("hostname").setValue(hostname);
        arg.get("port").setValue(port);

        //connect to remote process by socket
        return connector.attach(arg);
    }

获得VirtualMachine 后，您可以使用classesByName 和instances 方法获取类的实例。它返回 ReferenceType 的列表：

         VirtualMachine vm = attach("localhost", "56856");

        //get all classes of java.lang.String. There would be only one element.
        List<ReferenceType> classes = vm.classesByName("java.lang.String");

        //get all instances of a classes (set maximum count of instannces to get).
        List<ObjectReference> o = classes.get(0).instances(100000);

        //objectReference holds referenct to remote object. 
        for (ObjectReference objectReference : o) {
            try {
                //show text representation of remote object
                System.out.println(objectReference.toString());
            } catch (com.sun.jdi.ObjectCollectedException e) {
                //the specified object has been garbage collected
                //to avoid this use vm.suspend() vm.resume()
                System.out.println(e);
            }
        }

这是一个运行和连接的程序的工作示例自身并列出 java.lang.String 的所有实例。要运行示例，您需要类路径中 jdk 中的 tool.jar。

As explained in the other answers you can do it by using the JDI protocol. It is rather simple: you need to run the JVM in debug mode using

--agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=56855

after that, you can connect to the remote (or local JVM) and list all instances of the specified class. Also, you cannot directly cast remote objects to the real object but you can access to all the fields of the remote object and even cal methods.

Here how you can connect to remote JVM and get VirtualMachine.

    private static VirtualMachine attach(String hostname, String port) throws IOException, IllegalConnectorArgumentsException {
        //getSocketAttaching connector to connect to other JVM using Socket
        AttachingConnector connector = Bootstrap.virtualMachineManager().attachingConnectors()
                .stream().filter(p -> p.transport().name().contains("socket"))
                .findFirst().get();

        //set the arguments for the connector
        Map<String, Argument> arg = connector.defaultArguments();
        arg.get("hostname").setValue(hostname);
        arg.get("port").setValue(port);

        //connect to remote process by socket
        return connector.attach(arg);
    }

After getting VirtualMachine you can get instances of a class using methods classesByName and instances. It return List of ReferenceType:

         VirtualMachine vm = attach("localhost", "56856");

        //get all classes of java.lang.String. There would be only one element.
        List<ReferenceType> classes = vm.classesByName("java.lang.String");

        //get all instances of a classes (set maximum count of instannces to get).
        List<ObjectReference> o = classes.get(0).instances(100000);

        //objectReference holds referenct to remote object. 
        for (ObjectReference objectReference : o) {
            try {
                //show text representation of remote object
                System.out.println(objectReference.toString());
            } catch (com.sun.jdi.ObjectCollectedException e) {
                //the specified object has been garbage collected
                //to avoid this use vm.suspend() vm.resume()
                System.out.println(e);
            }
        }

Here is a working example of a program than runs and connects to itself and list all instances of a java.lang.String. To run an example you need tool.jar from jdk in you classpath.

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手心的温暖 2024-08-22 12:47:13

当我读到这篇文章时，我想一定有某种方法可以获取此类信息，因为 Java 分析器已经存在。也许这会有所帮助： http://java.sun .com/j2se/1.4.2/docs/guide/jvmpi/jvmpi.html。它描述了 JVM 和探查器代理之间的接口。但如果您确实想用 Java 编写此代码，那么您可能就不那么幸运了。

具体来说，查看这个函数：

jint (*EnableEvent)(jint event_type, void *arg);

    Called by the profiler agent to enable notification of a particular type of event. Apart from event_type, the profiler may also pass an argument that provides additional information specific to the given event type.

    All events are disabled when the VM starts up. Once enabled, an event stays enabled until it is explicitly disabled.

    This function returns JVMPI_NOT_AVAILABLE if event_type is JVMPI_EVENT_HEAP_DUMP, JVMPI_EVENT_MONITOR_DUMP or JVMPI_EVENT_OBJECT_DUMP. The profiler agent must use the RequestEvent function to request these events.

    Arguments:

        event_type  - type of event, JVMPI_EVENT_CLASS_LOAD etc.
        arg     - event specific argument.

    Returns:

        JVMPI_SUCCESS   enable succeeded.
        JVMPI_FAIL  enable failed.
        JVMPI_NOT_AVAILABLE     support for enabling the given event_type is not available.

When I read this I was thinking that there must be SOME way to get this kind of info, since java profilers exist. Maybe this will help: http://java.sun.com/j2se/1.4.2/docs/guide/jvmpi/jvmpi.html. It describes the interface between the JVM and a profiler agent. But if you were actually looking to write this in Java you may be out of luck.

Specifically, check out this function:

jint (*EnableEvent)(jint event_type, void *arg);

    Called by the profiler agent to enable notification of a particular type of event. Apart from event_type, the profiler may also pass an argument that provides additional information specific to the given event type.

    All events are disabled when the VM starts up. Once enabled, an event stays enabled until it is explicitly disabled.

    This function returns JVMPI_NOT_AVAILABLE if event_type is JVMPI_EVENT_HEAP_DUMP, JVMPI_EVENT_MONITOR_DUMP or JVMPI_EVENT_OBJECT_DUMP. The profiler agent must use the RequestEvent function to request these events.

    Arguments:

        event_type  - type of event, JVMPI_EVENT_CLASS_LOAD etc.
        arg     - event specific argument.

    Returns:

        JVMPI_SUCCESS   enable succeeded.
        JVMPI_FAIL  enable failed.
        JVMPI_NOT_AVAILABLE     support for enabling the given event_type is not available.

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嗫嚅 2024-08-22 12:47:13

http://java.sun.com/ j2se/1.5.0/docs/guide/jvmti/jvmti.html#IterateOverInstancesOfClass

您可以编写一些本机代码来获取 JVMTI 指针，然后使用它来
迭代给定类的所有实例，如上面的链接所示。
您可以从 Java 程序中调用此本机代码。
不过，正如 Eli 指出的那样，从 Java 6 开始，有一个名为 Java 调试接口的高级包装器，它允许您从 Java 本身进行此类调用，而不必使用本机代码。

希望这对

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