JVM常见问题有哪些

这篇文章主要介绍JVM常见问题有哪些，文中介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们一定要看完！

1. 多个 java 程序设置内存超过系统内存范围会阻止启动吗？即程序在启动时就占满其内存，还是按需增长？ 例如：一台主机，能启动5个默认配置（By default, the JVM will use MaxMemory/4 ）的程序吗？ 一同事说在一个128G内存的机器上无法启动新的java程序，询问得知此服务器启动过四个java应用，后查看服务器内存占满包括swap。

2. JVM 在什么时候会释放内存给操作系统？或者什么时候 JVM 进程内存使用量会降低 理论上 JVM "不会（或者说极为严苛）" 释放不使用的内存给 操作系统， 监控发现 JVM 进程内存使用量会降低，释放的是哪块内存 https://www.geekyhacker.com/2019/01/04/jvm-does-not-release-memory/ https://stackoverflow.com/questions/6785754/jvm-process-vs-jvm-heap-memory-usage

3. 如果主机内存资源紧张，会有抑制JVM虚拟机内存增长的机制吗？ 线上同一个服务有多个实例，在内存资源紧张的主机上的实例占用的内存要低于其他实例

4. java 内存与 Linux 显示进程使用内存的关系

jstat https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jstat.html Native Memory Tracking https://docs.oracle.com/javase/8/docs/technotes/guides/troubleshoot/tooldescr007.html jmx command tool emjmxcli

5. java 内存与 运行在 Docker 中 Linux 显示进程使用内存，Docker contener 使用内存的关系

6. java 内存与 运行在 kubernetes 中 Linux 显示进程使用内存，Docker contener 使用内存， POD 使用内存的关系

cat << \EOF > JmxTest.javaimport javax.management.*;import javax.management.remote.JMXServiceURL;import javax.management.remote.JMXConnectorFactory;import javax.management.remote.JMXConnector;import java.lang.management.RuntimeMXBean;import static java.lang.management.ManagementFactory.*;import java.net.Socket;import java.net.InetSocketAddress;import java.io.File;import java.io.IOException;import java.util.Properties;// Sun specificimport com.sun.tools.attach.VirtualMachine;// Sun implementation specificimport sun.management.ConnectorAddressLink;public class JmxTest {    /*     * Starts the management agent in the target VM     */    private static void startManagementAgent(String pid) throws IOException {        /*         * JAR file normally in ${java.home}/jre/lib but may be in ${java.home}/lib         * with development/non-images builds         */        String home = System.getProperty("java.home");        String agent = home + File.separator + "jre" + File.separator + "lib"                + File.separator + "management-agent.jar";        File f = new File(agent);        if (!f.exists()) {            agent = home + File.separator + "lib" + File.separator +                "management-agent.jar";            f = new File(agent);            if (!f.exists()) {                throw new RuntimeException("management-agent.jar missing");            }        }        agent = f.getCanonicalPath();        System.out.println("Loading " + agent + " into target VM ...");        try {            VirtualMachine.attach(pid).loadAgent(agent);        } catch (Exception x) {            throw new IOException(x.getMessage());        }    }    private static void connect(String pid, String address) throws Exception {        if (address == null) {            throw new RuntimeException("Local connector address for " +                                       pid + " is null");        }        System.out.println("Connect to process " + pid + " via: " + address);        JMXServiceURL url = new JMXServiceURL(address);        JMXConnector c = JMXConnectorFactory.connect(url);        MBeanServerConnection server = c.getMBeanServerConnection();        System.out.println("Connected.");        ObjectName directName = ObjectName.getInstance("java.nio:type=BufferPool,name=direct");        MBeanInfo mbInfo = server.getMBeanInfo(directName) ;        for(MBeanAttributeInfo i : mbInfo.getAttributes()) {            System.out .println(i.getName() + ":" + server.getAttribute(directName , i.getName()));        }//        RuntimeMXBean rt = newPlatformMXBeanProxy(server,//            RUNTIME_MXBEAN_NAME, RuntimeMXBean.class);//        System.out.println(rt.getName());        // close the connection        c.close();    }    private final static String LOCAL_CONNECTOR_ADDRESS_PROP =        "com.sun.management.jmxremote.localConnectorAddress";                    public static void main(String[] args) throws Exception {        String pid =  "1";        VirtualMachine vm = VirtualMachine.attach(pid);        String agentPropLocalConnectorAddress = (String)            vm.getAgentProperties().get(LOCAL_CONNECTOR_ADDRESS_PROP);        int vmid = Integer.parseInt(pid);        String jvmstatLocalConnectorAddress =            ConnectorAddressLink.importFrom(vmid);        if (agentPropLocalConnectorAddress == null &&            jvmstatLocalConnectorAddress == null) {            // No JMX Connector address so attach to VM, and load            // management-agent.jar            startManagementAgent(pid);            agentPropLocalConnectorAddress = (String)                vm.getAgentProperties().get(LOCAL_CONNECTOR_ADDRESS_PROP);            jvmstatLocalConnectorAddress =                ConnectorAddressLink.importFrom(vmid);        }        // Test address obtained from agent properties        System.out.println("Testing the connector address from agent properties");        connect(pid, agentPropLocalConnectorAddress);        // Test address obtained from jvmstat buffer//        System.out.println("Testing the connector address from jvmstat buffer");//        connect(pid, jvmstatLocalConnectorAddress);//        // Shutdown application//        int port = Integer.parseInt(args[1]);//        System.out.println("Shutdown process via TCP port: " + port);//        Socket s = new Socket();//        s.connect(new InetSocketAddress(port));//        s.close();    }}EOFjavac -Djava.ext.dirs=/usr/java/jdk1.8.0_121/lib JmxTest.javajava -Djava.ext.dirs=/usr/java/jdk1.8.0_121/lib JmxTest

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