Different Ways to Capture Java Heap Dumps – 捕获Java堆转储的不同方法

最后修改: 2018年 9月 20日

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1. Introduction

1.介绍

In this tutorial, we’ll explore different ways to capture a heap dump in Java.

在本教程中，我们将探讨在Java中捕获堆转储的不同方法。

A heap dump is a snapshot of all the objects that are in memory in the JVM at a certain moment. They are very useful to troubleshoot memory-leak problems and optimize memory usage in Java applications.

堆转储是某一时刻JVM中所有内存中对象的快照。它们对于排查内存泄漏问题和优化Java应用程序中的内存使用非常有用。

Heap dumps are usually stored in binary format hprof files. We can open and analyze these files using tools like jhat or JVisualVM. Also, for Eclipse users, it’s very common to use MAT.

堆转储通常以二进制格式的hprof文件存储。我们可以使用jhat或JVisualVM等工具打开并分析这些文件。另外，对于Eclipse用户来说，使用MAT是非常普遍的。

In the next sections, we’ll go through multiple tools and approaches to generate a heap dump, and we’ll show the main differences between them.

在接下来的章节中，我们将通过多种工具和方法来生成堆转储，我们将展示它们之间的主要区别。

2. JDK Tools

2.JDK工具

The JDK comes with several tools to capture heap dumps in different ways. All these tools are located under the bin folder inside the JDK home directory. Therefore, we can start them from the command line as long as this directory is included in the system path.

JDK 附带了几个工具，以不同的方式捕获堆转储。所有这些工具都位于JDK主目录下的bin文件夹中。因此，只要在系统路径中包含这个目录，我们就可以从命令行中启动它们。

In the next sections, we’ll look at how to use these tools to capture heap dumps.

在接下来的章节中，我们将看看如何使用这些工具来捕获堆转储。

2.1. jmap

2.1.jmap

jmap is a tool to print statistics about memory in a running JVM. We can use it for local or remote processes.

jmap是一个用于打印运行中的JVM的内存统计的工具。我们可以对本地或远程进程使用它。

To capture a heap dump using jmap, we need to use the dump option:

要使用jmap捕获一个堆转储，我们需要使用dump选项：。

jmap -dump:[live],format=b,file=<file-path> <pid>

Along with that option, we should specify several parameters:

与该选项一起，我们应该指定几个参数。

  • live: if set, it only prints objects which have active references and discards the ones that are ready to be garbage collected. This parameter is optional.
  • format=b: specifies that the dump file will be in binary format. If not set, the result is the same.
  • file: the file where the dump will be written to
  • pid: id of the Java process

An example would look like:

一个例子是这样的。

jmap -dump:live,format=b,file=/tmp/dump.hprof 12587

Remember that we can easily get the pid of a Java process by using the jps command.

请记住，我们可以通过使用jps命令轻松获得一个Java进程的pid。

Also, keep in mind that jmap was introduced in the JDK as an experimental tool and is unsupported. Therefore, in some cases, it may be preferable to use other tools instead.

另外，请记住， jmap在JDK中是作为一个实验性工具引入的，不受支持。因此，在某些情况下，使用其他工具可能更合适。

2.2. jcmd

2.2. jcmd

jcmd is a very complete tool that works by sending command requests to the JVM. We have to use it in the same machine where the Java process is running.

jcmd是一个非常完整的工具，它通过向JVM发送命令请求来工作。我们必须在运行Java进程的同一台机器上使用它。

One of its many commands is the GC.heap_dump. We can use it to get a heap dump just by specifying the pid of the process and the output file path:

它的众多命令之一是 GC.heap_dump。我们只需指定进程的pid和输出文件路径，就可以用它来获得堆转储。

jcmd <pid> GC.heap_dump <file-path>

We can execute it with the same parameters that we used before:

我们可以用之前使用的相同参数来执行它。

jcmd 12587 GC.heap_dump /tmp/dump.hprof

As with jmap, the dump generated is in binary format.

与jmap一样，生成的转储是二进制格式。

2.3. JVisualVM

2.3.JVisualVM

JVisualVM is a tool with a graphical user interface that lets us monitor, troubleshoot, and profile Java applications. The GUI is simple, but very intuitive and easy to use.

JVisualVM是一个具有图形用户界面的工具，可以让我们对Java应用程序进行监控、故障排除和剖析。GUI很简单，但非常直观，易于使用。

One of its many options allows us to capture a heap dump. If we right-click on a Java process and select the “Heap Dump” option, the tool will create a heap dump and open it in a new tab:

它的许多选项之一允许我们捕获一个堆转储。如果我们右击一个Java进程并选择“堆转储”选项，该工具将创建一个堆转储并在一个新的标签中打开。

dump menu cropped 1

Notice that we can find the path of the file created in the “Basic Info” section.

注意，我们可以在“基本信息”部分找到创建的文件的路径。

Starting from JDK 9, Visual VM isn’t included in the Oracle JDK and Open JDK distributions. Therefore, if we’re using anything newer than Java 9, we can get the JVisualVM from the Visual VM open source project site.

从JDK 9开始，Visual VM不包括在Oracle JDK和Open JDK发行版中。因此，如果我们使用的是比Java 9更新的东西，我们可以从Visual VM的开源项目网站获得JVisualVM。

3. Capture a Heap Dump Automatically

3.自动捕获堆数据

。

All the tools we’ve shown in the previous sections are intended to capture heap dumps manually at a specific time. In some cases, we want to get a heap dump when a java.lang.OutOfMemoryError occurs to help us investigate the error.

我们在前几节中展示的所有工具都是为了在特定时间手动捕获堆转储。在某些情况下，我们想在java.lang.OutOfMemoryError发生时获得堆转储，以帮助我们调查该错误。

For these cases, Java provides the HeapDumpOnOutOfMemoryError command-line option, which generates a heap dump when a java.lang.OutOfMemoryError is thrown:

对于这些情况，Java提供了HeapDumpOnOutOfMemoryError命令行选项，当java.lang.OutOfMemoryError被抛出时，它会生成一个堆转储：。

java -XX:+HeapDumpOnOutOfMemoryError

By default, it stores the dump in a java_pid<pid>.hprof file in the directory where we’re running the application. If we want to specify another file or directory, we can set it in the HeapDumpPath option:

默认情况下，它将转储存储在我们运行应用程序的目录中的java_pid<pid>.hprof文件中。如果我们想指定另一个文件或目录，我们可以在HeapDumpPath选项中设置：

java -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=<file-or-dir-path>

When our application runs out of memory using this option, we’ll be able to find the created file that contains the heap dump in the logs:

当我们的应用程序使用这个选项耗尽内存时，我们就能在日志中找到包含堆转储的创建文件。

java.lang.OutOfMemoryError: Requested array size exceeds VM limit
Dumping heap to java_pid12587.hprof ...
Exception in thread "main" Heap dump file created [4744371 bytes in 0.029 secs]
java.lang.OutOfMemoryError: Requested array size exceeds VM limit
	at com.baeldung.heapdump.App.main(App.java:7)

In the above example, it was written to the java_pid12587.hprof file.

在上述例子中，它被写入java_pid12587.hprof文件。

As we can see, this option is very useful, and there’s no overhead when running an application with this option. Therefore, it’s highly recommended to always use this option, especially in production.

正如我们所看到的，这个选项非常有用，而且使用这个选项运行应用程序时没有任何开销。因此，我们强烈建议始终使用这个选项，尤其是在生产中。

Finally, this option can also be specified at runtime by using the HotSpotDiagnostic MBean. To do this, we can use JConsole and set the HeapDumpOnOutOfMemoryError VM option to true:

最后，这个选项也可以通过使用HotSpotDiagnostic MBean在运行时指定。要做到这一点，我们可以使用JConsole并将HeapDumpOnOutOfMemoryError VM选项设置为true。

jconsole setvmoption 1

We can find more information about MBeans and JMX in this article.

我们可以在这个文章中找到更多关于MBeans和JMX的信息。

4. JMX

4.JMX[/strong]

The last approach we’ll cover in this article is using JMX. We’ll use the HotSpotDiagnostic MBean that we briefly introduced in the previous section. This MBean provides a dumpHeap method that accepts two parameters:

我们在本文中要介绍的最后一种方法是使用JMX。我们将使用我们在上一节简要介绍的HotSpotDiagnostic MBean。这个MBean提供了一个dumpHeap方法，它接受两个参数。

  • outputFile: the path of the file for the dump. This file should have the hprof extension.
  • live: if set to true, it dumps only the active objects in memory, as we saw before with jmap.

In the next sections, we’ll show two different ways to invoke this method in order to capture a heap dump.

在接下来的章节中，我们将展示调用该方法的两种不同方式，以捕获堆转储。

4.1. JConsole

4.1.JConsole

The easiest way to use the HotSpotDiagnostic MBean is by using a JMX client, such as JConsole.

使用HotSpotDiagnostic MBean的最简单方法是使用JMX客户端，如JConsole。

If we open JConsole and connect to a running Java process, we can navigate to the MBeans tab and find the HotSpotDiagnostic under com.sun.management. In operations, we can find the dumpHeap method that we previously described:

如果我们打开JConsole并连接到一个正在运行的Java进程，我们可以导航到MBeans标签并在com.sun.management下找到HotSpotDiagnostic。在操作中，我们可以找到我们之前描述的dumpHeap方法。

jconsole dump 1

As shown, we just need to introduce the parameters, outputFile and live, into the p0 and p1 text fields in order to perform the dumpHeap operation.

如图所示，我们只需要将参数outputFile和live,引入p0和p1文本字段，以便执行dumpHeap操作。

4.2. Programmatic Way

4.2.程序化方式

The other way to use the HotSpotDiagnostic MBean is by invoking it programmatically from Java code.

使用HotSpotDiagnostic MBean的另一种方式是通过从Java代码中以编程方式调用它。

To do this, we first need to get an MBeanServer instance in order to get an MBean that’s registered in the application. After that, we simply need to get an instance of a HotSpotDiagnosticMXBean, and call its dumpHeap method.

要做到这一点，我们首先需要获得一个MBeanServer实例，以便获得一个在应用程序中注册的MBean。之后，我们只需要获得一个HotSpotDiagnosticMXBean的实例，并调用其dumpHeap方法。

Let’s see it in code:

让我们看看它的代码。

public static void dumpHeap(String filePath, boolean live) throws IOException {
    MBeanServer server = ManagementFactory.getPlatformMBeanServer();
    HotSpotDiagnosticMXBean mxBean = ManagementFactory.newPlatformMXBeanProxy(
      server, "com.sun.management:type=HotSpotDiagnostic", HotSpotDiagnosticMXBean.class);
    mxBean.dumpHeap(filePath, live);
}

Notice that an hprof file cannot be overwritten. Therefore, we should take this into account when creating an application that prints heap dumps. If we fail to do so, we’ll get an exception:

注意，hprof文件不能被覆盖。因此，我们在创建打印堆转储的应用程序时应该考虑到这一点。如果我们没有这样做，我们会得到一个异常。

Exception in thread "main" java.io.IOException: File exists
	at sun.management.HotSpotDiagnostic.dumpHeap0(Native Method)
	at sun.management.HotSpotDiagnostic.dumpHeap(HotSpotDiagnostic.java:60)

5. Conclusion

5.结论

In this article, we learned multiple ways to capture a heap dump in Java.

在这篇文章中，我们学习了在Java中捕获堆转储的多种方法。

As a rule of thumb, we should always remember to use the HeapDumpOnOutOfMemoryError option when running Java applications. For different purposes, any of the other tools can be used, as long as we keep in mind the unsupported status of jmap.

作为一个经验法则，在运行Java应用程序时，我们应该始终记得使用HeapDumpOnOutOfMemoryError选项。对于不同的目的，可以使用其他任何工具，只要我们牢记jmap的不支持状态。

As always, the full source code of the examples is available over on GitHub.

一如既往，这些示例的完整源代码可在GitHub上获得。