Java SE 20 故障排查指南

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"Oracle公司发布的Java Platform, Standard Edition 20的故障排查指南" 《Java Platform, Standard Edition Troubleshooting Guide》是Oracle公司为Java SE 20版本提供的一份详尽的技术文档，旨在帮助开发者、系统管理员和故障排除专家解决在使用Java SE平台时可能遇到的各种问题。该指南涵盖了从基础的环境配置到复杂的运行时错误等多种场景，是Java开发者的重要参考资料。此文档的内容可能包括以下几个主要方面： 1. **环境设置与配置**：详细介绍了如何正确安装和配置Java Development Kit (JDK) 20，包括设置环境变量如JAVA_HOME、PATH等，以及确保JDK的版本兼容性和系统需求。 2. **编译与运行问题**：针对Java源代码的编译错误和运行时异常，提供了诊断和解决方法，例如类路径设置错误、依赖库冲突、内存溢出等问题。 3. **性能优化**：指导用户如何分析和改善Java应用程序的性能，如使用JVisualVM或JProfiler进行性能监控，调整JVM参数以优化内存分配和垃圾回收。 4. **多线程与并发问题**：解释了在并发编程中可能出现的死锁、竞态条件和线程安全问题，并提供了相应的调试工具和最佳实践。 5. **网络与I/O问题**：针对网络通信和输入输出可能出现的问题，如连接失败、读写错误，提供了解决方案和排查步骤。 6. **安全与权限**：详细阐述了Java安全模型，如何处理权限问题，如签名的jar文件、访问控制策略和Java安全管理器的配置。 7. **日志与调试**：指导如何利用Java的内置日志框架，如java.util.logging，以及如何使用第三方日志库，进行问题定位。同时，介绍了如何使用JDB、Java Mission Control (JMC)等工具进行调试。 8. **模块系统**：对于Java 9引入的模块系统，可能会遇到的模块间的依赖问题和模块化应用的部署问题，提供了相应的故障排查策略。 9. **Java Native Interface (JNI)**：如果程序涉及到本地方法调用，此指南将提供关于JNI配置和使用中的常见问题及其解决方案。 10. **兼容性与升级**：讨论了不同Java版本之间的兼容性问题，以及从旧版本升级到Java 20时需要注意的事项。《Java Platform, Standard Edition Troubleshooting Guide》是一个全面的故障排查手册，不仅包含了Java SE 20的特性和改进，还提供了丰富的案例和实践建议，对于Java开发者来说是不可或缺的工具。由于文档版权的限制，这里仅提供了部分可能涵盖的内容概述，具体的故障排查步骤和技术细节需参考原文档以获取完整信息。

Note:

Core files may take up a lot of disk space depending on the memory

usage of the application.

Core files provide detailed information about the state of an application at the time

of a crash. They're not always useful in diagnosing a problem but are invaluable

when they are needed. Because crashes are often difficult to reproduce, having

application core files enabled can save time and provide valuable information. It's

up to your and your organization to decide whether your system can meet the

potential space requirements.

2. Add -XX:+HeapDumpOnOutOfMemoryError to the JVM flags: The

XX:+HeapDumpOnOutOfMemoryError

flag saves a Java heap dump to disk if the

applications runs into an

OutOfMemoryError

A heap dump provides information on that portion of an application's memory

usage that is managed by the JVM directly. Like core files, heap dumps can be

very large. A heap dump can identify if part of an application is growing out of

proportion to the rest, which may cause an OutOfMemoryError.

3. Run a continuous flight recording: Set up Java to run with a continuous flight

recording.

Continuous flight recordings provide a file-based record of JVM events. This

provides a low-overhead means of monitoring a Java application that can be used

in the event of an issue. See Produce a Flight Recording for how to setup such

recordings.

4. Add the -Xlog:gc* command-line option: The

-Xlog:gc

option prints detailed

information about garbage collection, which helps diagnose problems with

application performance and memory management.

See Enable Logging with the JVM Unified Logging Framework in the

java

command documentation for more information about this option and other

information that the JVM Unified Logging Framework can print.

Note:

Creating a discrete garbage collection log file makes it easier to read. It

also persists after a crash or application restart. Setting up log rotation

enables you to manage the amount of information retained and for how

long. See -Xlog Output in the

java

command documentation for how to

do this.

5. Print Java version and JVM flags: Oracle Support, and even forums, ask for the

exact Java version your application uses and its JVM flags.

Run

java -version

at a command line to obtain the default Java version of your

system. If your application starts with a script, read through it and any associated

configuration files to find out if your application is using something other than the

default Java version. Alternatively, add

-XX:PrintFlagsFinal

and

-showversion

to your application's JVM arguments, restart your application, and read the

application logs for the results.

Chapter 1

Enable Options and Flags for JVM Troubleshooting

1-2

In general, the diagnostic tools and options use various mechanisms to get the

information they report. The mechanisms are specific to the virtual machine (VM)

implementation, operating systems, and release. Frequently, only a subset of the tools

is applicable to a given issue at a particular time. Command-line options that are

prefixed with

-XX

are specific to Java HotSpot VM. See Java HotSpot VM Command-

Line Options.

Note:

The -XX options are not part of the Java API and can vary from one release

to the next.

The tools and options are divided into several categories, depending on the type of

problem that you are troubleshooting. Certain tools and options might fall into more

than one category.

• Postmortem diagnostics These tools and options can be used to diagnose a

problem after an application crashes. See Postmortem Diagnostic Tools.

• Hung processes These tools can be used to investigate a hung or deadlocked

process. See Hung Processes Tools.

• Monitoring These tools can be used to monitor a running application. See

Monitoring Tools.

• Other These tools and options can be used to help diagnose other issues. See

Other Tools, Options, Variables, and Properties.

Note:

Some command-line utilities described in this section are experimental. The

jstack

jinfo

, and

jmap

utilities are examples of utilities that are

experimental. It is suggested to use the latest diagnostic utility,

jcmd

instead

of the earlier

jstack

jinfo,

and

jmap

utilities.

JDK Mission Control

Java Platform, Standard Edition (JMC) is a production-time profiling and diagnostics

tool. It includes tools to monitor and manage your Java application with a very small

performance overhead, and is suitable for monitoring applications running in

production.

JMC is not part of the regular JDK installation. For more information on JMC

downloads and documentation, see JDK Mission Control Page.

JMC consists of :

• JVM Browser shows running Java applications and their JVMs.

• JMX Console is a mechanism for monitoring and managing JVMs. It connects to a

running JVM, collects, displays its characteristics in real time, and enables you to

Chapter 2

JDK Mission Control

2-2

change some of its runtime properties through Managed Beans (MBeans). You can also

create rules that trigger on certain events (for example, send an e-mail if the CPU usage

by the application reaches 90 percent).

• Flight Recorder (JFR) is a tool for collecting diagnostic and profiling data about a running

Java application. It is integrated into the JVM and causes very small performance

overhead, so it can be used in production environments. JFR continuously saves large

amounts of data about the running applications. This profiling information includes thread

samples, lock profiles, and garbage collection details. JFR presents diagnostic

information in logically grouped tables and charts. It enables you to select the range of

time and level of detail necessary to focus on the problem. Data collected by JFR can be

essential when contacting Oracle support to help diagnose issues with your Java

application.

• Plug-ins help in heap dump analysis and DTrace recording. See Plug-in Details. Java SE

plug-ins connect to a JVM using the Java Management Extensions (JMX) agent. For

more information about JMX, see the Java Platform, Standard Edition Java Management

Extensions Guide .

Troubleshoot with JDK Mission Control

JMC provides the following features or functionalities that can help you in troubleshooting:

• Java Management console (JMX) connects to a running JVM, and collects and displays

key characteristics in real time.

• Triggers user-provided custom actions and rules for JVM.

• Experimental plug-ins from the JMC tool provide troubleshooting activities.

• Flight Recording in JMC is available to analyze events. The preconfigured tabs enable

you to easily to drill down in various areas of common interest, such as, code, memory

and garbage collection, threads, and I/O. The Automated Analysis Results page of

flight recordings helps you to diagnose issues quicker. The provided rules and heuristics

help you find functional and performance problems in your application and provide tuning

tips. Some rules that operate with relatively unknown concepts, like safe points, will

provide explanations and links to further information. Some rules are parametrized and

can be configured to make more sense in your particular environment. Individual rules

can be enabled or disabled as you see fit.

– Flight Recorder in the JMC application presents diagnostic information in logically

grouped tables, charts, and dials. It enables you to select the range of time and level

of detail necessary to focus on the problem.

• The JMC plug-ins connect to JVM using the Java Management Extensions (JMX) agent.

The JMX is a standard API for the management and monitoring of resources such as

applications, devices, services, and the Java Virtual Machine.

Flight Recorder

Flight Recorder (JFR) is a profiling and event collection framework built into the JDK.

Flight Recorder allows Java administrators and developers to gather detailed low-level

information about how a JVM and Java applications are behaving. You can use JMC to

visualize the data collected by JFR. Flight Recorder and JMC together create a complete

toolchain to continuously collect low-level and detailed runtime information enabling after-the-

fact incident analysis.

Chapter 2

Flight Recorder

2-3

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tar -zxvf /opt/software/jdk-8u311-linux-x64.tar.gz -C /opt/module/ tar (child): /opt/software/jdk-8u311-linux-x64.tar.gz：无法 open: 没有那个文件或目录

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