系列

• Arthas入门篇
• Arthas功能介绍
• Arthas 启动过程分析
• Arthas使用Idea调试
• Arthas Command处理流程
• Arthas类查找和反编译原理
• Arthas内存动态编译原理
• Arthas动态重新加载类
• Arthas导出加载类
• Arthas classloader类加载器
• Arthas 查看当前JVM信息
• Arthas thread查看线程信息

开篇

• Arthas提供sysprop命令查看当前JVM的系统属性(System Property)。
• Arthas提供sysenv查看当前JVM的环境属性(System Environment Variables)。
• Arthas提供vmoption查看更新VM诊断相关的参数。

原理介绍

在安装JDK的时候需要配置环境变量，安装完之后我们需要查看JDK的版本，我们就需要使用查看系统属性。Java提供了System类的静态方法getenv()方法和getProperty()方法，用于返回系统相关的环境变量与系统属性。

• getenv()方法返回的变量大多与操作系统相关。
• getProperty()方法返回的变量大多与Java程序有关。
• 通过HotSpotDiagnosticMXBean的getVMOption来获取VMOption参数。

源码分析

public class SystemEnvCommand extends AnnotatedCommand {

    private String envName;

    @Argument(index = 0, argName = "env-name", required = false)
    @Description("env name")
    public void setOptionName(String envName) {
        this.envName = envName;
    }

    @Override
    public void process(CommandProcess process) {
        try {
            SystemEnvModel result = new SystemEnvModel();
            if (StringUtils.isBlank(envName)) {
                // show all system env
                result.putAll(System.getenv());
            } else {
                // view the specified system env
                String value = System.getenv(envName);
                result.put(envName, value);
            }
            process.appendResult(result);
            process.end();
        } catch (Throwable t) {
            process.end(-1, "Error during setting system env: " + t.getMessage());
        }
    }
}

• System.getenv()来获取环境变量。

public class SystemPropertyCommand extends AnnotatedCommand {

    private String propertyName;
    private String propertyValue;

    @Override
    public void process(CommandProcess process) {
        try {

            if (StringUtils.isBlank(propertyName) && StringUtils.isBlank(propertyValue)) {
                // show all system properties
                process.appendResult(new SystemPropertyModel(System.getProperties()));
            } else if (StringUtils.isBlank(propertyValue)) {
                // view the specified system property
                String value = System.getProperty(propertyName);
                if (value == null) {
                    process.end(1, "There is no property with the key " + propertyName);
                    return;
                } else {
                    process.appendResult(new SystemPropertyModel(propertyName, value));
                }
            } else {
                // change system property
                System.setProperty(propertyName, propertyValue);
                process.appendResult(new MessageModel("Successfully changed the system property."));
                process.appendResult(new SystemPropertyModel(propertyName, System.getProperty(propertyName)));
            }
            process.end();
        } catch (Throwable t) {
            process.end(-1, "Error during setting system property: " + t.getMessage());
        }
    }
}

• 通过System.getProperty来获取属性。

public class VMOptionCommand extends AnnotatedCommand {

    private String name;
    private String value;

    private static void run(CommandProcess process, String name, String value) {
        try {
            HotSpotDiagnosticMXBean hotSpotDiagnosticMXBean = ManagementFactory
                            .getPlatformMXBean(HotSpotDiagnosticMXBean.class);

            if (StringUtils.isBlank(name) && StringUtils.isBlank(value)) {
                // show all options
                process.appendResult(new VMOptionModel(hotSpotDiagnosticMXBean.getDiagnosticOptions()));
            } else if (StringUtils.isBlank(value)) {
                // view the specified option
                VMOption option = hotSpotDiagnosticMXBean.getVMOption(name);
                if (option == null) {
                    process.end(-1, "In order to change the system properties, you must specify the property value.");
                    return;
                } else {
                    process.appendResult(new VMOptionModel(Arrays.asList(option)));
                }
            } else {
                VMOption vmOption = hotSpotDiagnosticMXBean.getVMOption(name);
                String originValue = vmOption.getValue();

                // change vm option
                hotSpotDiagnosticMXBean.setVMOption(name, value);
                process.appendResult(new MessageModel("Successfully updated the vm option."));
                process.appendResult(new VMOptionModel(new ChangeResultVO(name, originValue,
                        hotSpotDiagnosticMXBean.getVMOption(name).getValue())));
            }
            process.end();
        } catch (Throwable t) {
        }
    }
}

• 通过ManagementFactory.getPlatformMXBean(HotSpotDiagnosticMXBean.class)来获取HotSpotDiagnosticMXBean对象。
• 通过hotSpotDiagnosticMXBean.getDiagnosticOptions()来获取VMOption属性。