在游戏服务器的开发中，热更是一个比较常见的需求。比如说线上出现了一个Bug，如果没有热更的话，那么就需要在修复Bug之后重新打包发布到服务器上，然后重新启动。重启对于一个线上的游戏来说是损失比较大的。
实现思路如下：用一个线程监听存放class文件的目录，如果有class文件就读取，然后重新加载到内存中，完成之后再把这些class文件删除。此种实现方法只能修改方法体的内容，但是已经能满足大部分需求了。
具体实现代码：
package com.hotswap;
import java.lang.instrument.Instrumentation;
import java.util.logging.ConsoleHandler;
import java.util.logging.Logger;
/**
* class热替换
*
* <pre>
* 用于在应用程序运行期间，修改代码后，不用重启服务器，就能生效。
* 只有在修改方法体里的代码才生效，其他情况都不适用。
* </pre>
*
* @author zyb
*
* @date 2017年5月26日 下午5:33:15
*/
public class HotSwapAgent {
/** 默认扫描间隔时间 */
private static final int DEFAULT_SCAN_INTERVAL = 500;
private static final Logger log = Logger.getLogger(HotSwapAgent.class.getName());
private final Instrumentation instrumentation;
/** 要监视的class文件路径 */
private final String classPath;
public static void premain(String agentArgs, Instrumentation inst) {
init(agentArgs, inst);
}
public static void agentmain(String agentArgs, Instrumentation inst) {
init(agentArgs, inst);
}
private static void init(String agentArgs, Instrumentation inst) {
AgentArgs args = new AgentArgs(agentArgs);
if (!args.isValid()) {
throw new RuntimeException("args is invalid");
}
new HotSwapAgent(inst, args);
}
public HotSwapAgent(Instrumentation inst, AgentArgs args) {
this.instrumentation = inst;
this.classPath = args.getClassPath();
int scanInterval = DEFAULT_SCAN_INTERVAL;
if (args.getInterval() > scanInterval) {
scanInterval = args.getInterval();
}
log.setUseParentHandlers(false);
log.setLevel(args.getLogLevel());
ConsoleHandler consoleHandler = new ConsoleHandler();
consoleHandler.setLevel(args.getLogLevel());
log.addHandler(consoleHandler);
HotSwapMonitor monitor = new HotSwapMonitor(instrumentation, classPath, args.getInterval());
monitor.start();
log.info("class path: " + classPath);
log.info("scan interval (ms): " + scanInterval);
log.info("log level: " + log.getLevel());
}
}
package com.hotswap;
import java.io.File;
import java.util.HashMap;
import java.util.Map;
import java.util.logging.Level;
/**
* Agent参数
*
* <pre>
* 在程序启动时，命令行指定参数。
* 例：
* -javaagent:${path}/hotswap-agent-1.0.jar="classes=${classPath}, interval=1000, logLevel=FINE"
* hotswap-agent-1.0.jar就是本项目的jar文件
* classPath：用于指定要扫描的class文件所在的目录
* interval：扫描文件的时间间隔
* logLevel：日志级别，级别参照JDK的java.util.logging.Level
* </pre>
*
* @author zyb
*
* @date 2017年5月26日 下午7:42:19
*/
public class AgentArgs {
/** class文件的存放路径 */
private static final String CLASSES_PATH = "classPath";
/** 扫描间隔时间 */
private static final String SCAN_INTERVAL = "interval";
/** 日志级别 */
private static final String LOG_LEVEL = "logLevel";
private String classPath;
private int interval;
private Level logLevel;
private AgentArgs() {
this.classPath = null;
this.interval = -1;
this.logLevel = Level.WARNING;
}
public AgentArgs(String agentArgs) {
this();
if (agentArgs != null && agentArgs.length() > 0) {
if (agentArgs.indexOf("=") != -1) {
initArgs(agentArgs);
}
}
}
public String getClassPath() {
return classPath;
}
public Level getLogLevel() {
return logLevel;
}
public int getInterval() {
return interval;
}
private void initArgs(String agentArgs) {
String[] args = agentArgs.split(",");
Map<String, String> argsMap = new HashMap<String, String>();
for (String s : args) {
String[] param = s.split("=");
argsMap.put(param[0].trim(), param[1]);
}
if (argsMap.containsKey(CLASSES_PATH)) {
setClassPath(argsMap.get(CLASSES_PATH));
}
if (argsMap.containsKey(SCAN_INTERVAL)) {
setInterval(argsMap.get(SCAN_INTERVAL));
}
if (argsMap.containsKey(LOG_LEVEL)) {
setLogLevel(argsMap.get(LOG_LEVEL));
}
}
public boolean isValid() {
return classPath != null;
}
private void setClassPath(String classPath) {
this.classPath = parsePath(classPath);
}
private void setLogLevel(String logLevel) {
try {
this.logLevel = Level.parse(logLevel.trim());
} catch (Exception e) {
this.logLevel = Level.WARNING;
}
}
private void setInterval(String interval) {
try {
this.interval = Integer.parseInt(interval.trim());
} catch (NumberFormatException e) {
this.interval = -1;
}
}
private static String parsePath(String path) {
if (path != null) {
String result = path.trim();
return result.endsWith(File.separator) ? result : result + File.separator;
}
return null;
}
}
package com.hotswap;
import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.lang.instrument.ClassDefinition;
import java.lang.instrument.Instrumentation;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.logging.Level;
import java.util.logging.Logger;
import jdk.internal.org.objectweb.asm.ClassReader;
/**
* 热替换任务
*
* @author zyb
*
* @date 2017年6月1日 上午10:03:21
*/
public class HotSwapMonitor implements Runnable {
/** 要监视的目录 */
private String classPath;
private Instrumentation instrumentation;
private ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();
private int interval;
private static final Logger logger = Logger.getLogger(HotSwapMonitor.class.getName());
public HotSwapMonitor(Instrumentation instrumentation, String classPath, int interval) {
this.instrumentation = instrumentation;
this.classPath = classPath;
this.interval = interval;
}
public void start() {
logger.info("HotSwapMonitor start...");
executor.scheduleAtFixedRate(this, 0, interval, TimeUnit.MILLISECONDS);
}
@Override
public void run() {
try {
scanClassFile();
} catch (Exception e) {
logger.log(Level.SEVERE, "HotSwapMonitor error", e);
}
}
/**
* 扫描class文件
*/
public void scanClassFile() throws Exception {
File path = new File(classPath);
File[] files = path.listFiles();
if (files == null) {
return;
}
String classFilePath = null;
boolean success = false;
long now = System.currentTimeMillis();
for (File file : files) {
if (!isClassFile(file)) {
continue;
}
classFilePath = file.getPath();
reloadClass(classFilePath);
logger.fine(String.format("Reload %s success", classFilePath));
file.delete();
success = true;
}
if (success) {
logger.fine(String.format("Reload success, cost time:%sms", (System.currentTimeMillis() - now)));
}
}
/**
* 重新加载class
*
* @param classFilePath
*/
private void reloadClass(String classFilePath) throws Exception {
File file = new File(classFilePath);
byte[] buff = new byte[(int) file.length()];
DataInputStream in = new DataInputStream(new FileInputStream(file));
in.readFully(buff);
in.close();
FileInputStream fis = new FileInputStream(file);
ClassReader reader = new ClassReader(fis);
fis.close();
ClassDefinition definition = new ClassDefinition(Class.forName(reader.getClassName()), buff);
instrumentation.redefineClasses(new ClassDefinition[] { definition });
}
/**
* 是否class文件
*
* @param file
* @return
*/
private boolean isClassFile(File file) {
return file.getName().contains(".class");
}
}
build.xml
<project name="hotswap-agent" default="dist" basedir=".">
<property name="version" value="1.0" />
<property name="jar.name" value="${ant.project.name}-${version}" />
<property name="classes.dir" value="bin" />
<property name="jar.dir" value="jar" />
<property name="javac.version" value="1.5" />
<target name="prepare">
<mkdir dir="${classes.dir}" />
<mkdir dir="${jar.dir}" />
</target>
<target name="build" depends="prepare">
<javac target="${javac.version}" srcdir="src" debug="true" destdir="${classes.dir}">
<include name="**/*.java" />
<exclude name="**/com/hotswap/HotSwapAgent.java" />
</javac>
</target>
<target name="dist">
<antcall target="clean" />
<antcall target="build" />
<jar basedir="${classes.dir}" jarfile="${jar.dir}/${jar.name}.jar" manifest="src/META-INF/MANIFEST.MF" />
</target>
<target name="clean">
<delete dir="${classes.dir}" />
<delete dir="${jar.dir}" />
</target>
</project>
在src下新建META-INF目录，再新建MANIFEST.MF文件
Manifest-Version: 1.0 Agent-Class: com.hotswap.HotSwapAgent Premain-Class: com.hotswap.HotSwapAgent Can-Redefine-Classes: true
用ant将项目打包成jar，放到classpath下。然后在JVM启动参数中加入-javaagent:${path}/hotswap-agent-1.0.jar="classes=${classPath}, interval=1000, logLevel=FINE" ${xxx}表示一个变量，替换成自己的目录就行了,classes用于指定存放class文件的目录，将要热更的class文件放到此目录下即可。