Tomcat启动分析(一) - Bootstrap类

本系列以Tomcat 8.5.33为例分析Tomcat的启动过程。

Tomcat的启动脚本

与Tomcat有关的脚本都在Tomcat主目录的bin子目录中，其中与启动有关的脚本有startup.sh、catalina.sh和setclasspath.sh。启动Tomcat时只需执行startup.sh即可，其内容如下：

# resolve links - $0 may be a softlink
PRG="$0"

while [ -h "$PRG" ] ; do
  ls=`ls -ld "$PRG"`
  link=`expr "$ls" : '.*-> \(.*\)$'`
  if expr "$link" : '/.*' > /dev/null; then
    PRG="$link"
  else
    PRG=`dirname "$PRG"`/"$link"
  fi
done

PRGDIR=`dirname "$PRG"`
EXECUTABLE=catalina.sh

# Check that target executable exists
if $os400; then
  # -x will Only work on the os400 if the files are:
  # 1. owned by the user
  # 2. owned by the PRIMARY group of the user
  # this will not work if the user belongs in secondary groups
  eval
else
  if [ ! -x "$PRGDIR"/"$EXECUTABLE" ]; then
    echo "Cannot find $PRGDIR/$EXECUTABLE"
    echo "The file is absent or does not have execute permission"
    echo "This file is needed to run this program"
    exit 1
  fi
fi

exec "$PRGDIR"/"$EXECUTABLE" start "$@"

该脚本主要做了两件事：

• 用while循环处理符号链接，ls变量存ls命令的结果，link变量存符号链接指向的文件路径，直到PRG的值不再是符号链接为止，以此找到Tomcat的主目录；
• exec命令执行catalina.sh，命令行参数由start和传递给startup.sh的命令行参数两部分组成。

catalina.sh利用setclasspath.sh检验JDK或者JRE环境，然后根据传递给脚本的的命令行参数启动JVM，不同参数对应的分支代码如下，传给catalina.sh脚本的第一个命令行参数是给Bootstrap类传递的最后一个命令行参数。以默认情况为例，上文的startup.sh只为catalina.sh提供了start参数，则$@为空，因此会执行eval所示的代码。

if [ "$1" = "debug" ] ; then
  # 省略部分代码
elif [ "$1" = "run" ]; then
  # 省略部分代码
elif [ "$1" = "start" ] ; then
  # 省略部分代码
  shift
  touch "$CATALINA_OUT"
  if [ "$1" = "-security" ] ; then
    # 省略部分代码
  else
    eval $_NOHUP "\"$_RUNJAVA\"" "\"$LOGGING_CONFIG\"" $LOGGING_MANAGER $JAVA_OPTS $CATALINA_OPTS \
      -D$ENDORSED_PROP="\"$JAVA_ENDORSED_DIRS\"" \
      -classpath "\"$CLASSPATH\"" \
      -Dcatalina.base="\"$CATALINA_BASE\"" \
      -Dcatalina.home="\"$CATALINA_HOME\"" \
      -Djava.io.tmpdir="\"$CATALINA_TMPDIR\"" \
      org.apache.catalina.startup.Bootstrap "$@" start \
      >> "$CATALINA_OUT" 2>&1 "&"
  fi

  if [ ! -z "$CATALINA_PID" ]; then
    echo $! > "$CATALINA_PID"
  fi

  echo "Tomcat started."
elif [ "$1" = "stop" ] ; then
  # 省略部分代码
elif [ "$1" = "configtest" ] ; then
  # 省略部分代码
elif [ "$1" = "version" ] ; then
  # 省略部分代码
else
  echo "Usage: catalina.sh ( commands ... )"
  # 省略部分代码
fi

用ps aux | grep tomcat可以看到对应的执行命令如下

/Library/Java/JavaVirtualMachines/jdk1.8.0_131.jdk/Contents/Home/bin/java 
-Djava.util.logging.config.file=/Users/suntao/Documents/Workspace/apache-tomcat-8.5.33/conf/logging.properties
-Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager
-Djdk.tls.ephemeralDHKeySize=2048
-Djava.protocol.handler.pkgs=org.apache.catalina.webresources
-Dorg.apache.catalina.security.SecurityListener.UMASK=0027
-Dignore.endorsed.dirs= -classpath /Users/suntao/Documents/Workspace/apache-tomcat-8.5.33/bin/bootstrap.jar:/Users/suntao/Documents/Workspace/apache-tomcat-8.5.33/bin/tomcat-juli.jar
-Dcatalina.base=/Users/suntao/Documents/Workspace/apache-tomcat-8.5.33
-Dcatalina.home=/Users/suntao/Documents/Workspace/apache-tomcat-8.5.33
-Djava.io.tmpdir=/Users/suntao/Documents/Workspace/apache-tomcat-8.5.33/temp
org.apache.catalina.startup.Bootstrap start

• catalina.home是Tomcat的安装主目录；
• catalina.base是Tomcat的工作主目录，在多个Tomcat时与catalina.home不同；
• 主类是org.apache.catalina.startup.Bootstrap，命令行参数只有一个start，下面就分析Bootstrap类。

Bootstrap类

静态代码块

首先看静态代码块，该代码块重新设置了两个系统属性，catalina.home和catalina.base。

private static final File catalinaBaseFile;
private static final File catalinaHomeFile;

static {
    // Will always be non-null
    String userDir = System.getProperty("user.dir");

    // Home first
    String home = System.getProperty(Globals.CATALINA_HOME_PROP);
    File homeFile = null;

    if (home != null) {
        File f = new File(home);
        try {
            homeFile = f.getCanonicalFile();
        } catch (IOException ioe) {
            homeFile = f.getAbsoluteFile();
        }
    }

    if (homeFile == null) {
        // First fall-back. See if current directory is a bin directory
        // in a normal Tomcat install
        File bootstrapJar = new File(userDir, "bootstrap.jar");

        if (bootstrapJar.exists()) {
            File f = new File(userDir, "..");
            try {
                homeFile = f.getCanonicalFile();
            } catch (IOException ioe) {
                homeFile = f.getAbsoluteFile();
            }
        }
    }

    if (homeFile == null) {
        // Second fall-back. Use current directory
        File f = new File(userDir);
        try {
            homeFile = f.getCanonicalFile();
        } catch (IOException ioe) {
            homeFile = f.getAbsoluteFile();
        }
    }

    catalinaHomeFile = homeFile;
    System.setProperty(
            Globals.CATALINA_HOME_PROP, catalinaHomeFile.getPath());

    // Then base
    String base = System.getProperty(Globals.CATALINA_BASE_PROP);
    if (base == null) {
        catalinaBaseFile = catalinaHomeFile;
    } else {
        File baseFile = new File(base);
        try {
            baseFile = baseFile.getCanonicalFile();
        } catch (IOException ioe) {
            baseFile = baseFile.getAbsoluteFile();
        }
        catalinaBaseFile = baseFile;
    }
    System.setProperty(
            Globals.CATALINA_BASE_PROP, catalinaBaseFile.getPath());
}

看源码的过程也是学习的过程，user.dir可以获取进程的工作目录，具体各属性的含义可以参见Javadoc文档，以前没用过这个属性，好奇地看了一下实现。以下是System类的部分源码，getProperty函数会从props变量取值，initProperties这个JNI方法会去初始化props变量：

private static Properties props;
private static native Properties initProperties(Properties props);
public static String getProperty(String key) {
    checkKey(key);
    SecurityManager sm = getSecurityManager();
    if (sm != null) {
        sm.checkPropertyAccess(key);
    }

    return props.getProperty(key);
}

接下来看一下JNI方法initProperties是如何实现的：

• 下载openjdk-8的源码，在openjdk-8-source/jdk/src/share/native/java/lang/System.c中找到initProperties对应的JNI函数Java_java_lang_System_initProperties，其部分代码如下，GetJavaProperties(env)函数返回了获得的系统属性：

  JNIEXPORT jobject JNICALL
  Java_java_lang_System_initProperties(JNIEnv *env, jclass cla, jobject props)
  {
      char buf[128];
      java_props_t *sprops = GetJavaProperties(env);
      // 省略一些代码
  }
  

• java_props_t 结构体在openjdk-8-source/jdk/src/share/native/java/lang/java_props.h中定义，结构体的user_dir即为要获取的user.dir属性：

  #ifdef WIN32
  #include <tchar.h>
  typedef WCHAR nchar;
  #else
  typedef char nchar;
  #endif
  
  typedef struct {
      char *os_name;
      char *os_version;
      char *os_arch;
  
  #ifdef JDK_ARCH_ABI_PROP_NAME
      char *sun_arch_abi;
  #endif
  
      nchar *tmp_dir;
      nchar *font_dir;
      nchar *user_dir;
  
      char *file_separator;
      char *path_separator;
      char *line_separator;
  
      nchar *user_name;
      nchar *user_home;
  
      char *language;
      char *format_language;
      char *display_language;
      char *script;
      char *format_script;
      char *display_script;
      char *country;
      char *format_country;
      char *display_country;
      char *variant;
      char *format_variant;
      char *display_variant;
      char *encoding;
      char *sun_jnu_encoding;
      char *sun_stdout_encoding;
      char *sun_stderr_encoding;
      char *timezone;
  
      char *printerJob;
      char *graphics_env;
      char *awt_toolkit;
  
      char *unicode_encoding;     /* The default endianness of unicode
                                      i.e. UnicodeBig or UnicodeLittle   */
  
      const char *cpu_isalist;    /* list of supported instruction sets */
  
      char *cpu_endian;           /* endianness of platform */
  
      char *data_model;           /* 32 or 64 bit data model */
  
      char *patch_level;          /* patches/service packs installed */
  
      char *desktop;              /* Desktop name. */
      // 省略一些代码
  } java_props_t;
  
  java_props_t *GetJavaProperties(JNIEnv *env);
  

• GetJavaProperties函数在openjdk-8-source/jdk/src/solaris/native/java/lang/java_props_md.c中定义，其部分代码如下，这里看到了熟悉的getcwd函数，正是熟悉的APUE味道：

  /* Current directory */
  {
      char buf[MAXPATHLEN];
      errno = 0;
      if (getcwd(buf, sizeof(buf))  == NULL)
          JNU_ThrowByName(env, "java/lang/Error",
           "Properties init: Could not determine current working directory.");
      else
          sprops.user_dir = strdup(buf);
  }
  

init函数

Bootstrap类的init函数代码如下：

public void init() throws Exception {
    initClassLoaders();

    Thread.currentThread().setContextClassLoader(catalinaLoader);

    SecurityClassLoad.securityClassLoad(catalinaLoader);

    // Load our startup class and call its process() method
    if (log.isDebugEnabled())
        log.debug("Loading startup class");
    Class<?> startupClass = catalinaLoader.loadClass("org.apache.catalina.startup.Catalina");
    Object startupInstance = startupClass.getConstructor().newInstance();

    // Set the shared extensions class loader
    if (log.isDebugEnabled())
        log.debug("Setting startup class properties");
    String methodName = "setParentClassLoader";
    Class<?> paramTypes[] = new Class[1];
    paramTypes[0] = Class.forName("java.lang.ClassLoader");
    Object paramValues[] = new Object[1];
    paramValues[0] = sharedLoader;
    Method method = startupInstance.getClass().getMethod(methodName, paramTypes);
    method.invoke(startupInstance, paramValues);

    catalinaDaemon = startupInstance;
}

init函数主要做了以下两件事：

• 调用initClassLoaders方法初始化类加载器，并继续设置系统属性：
  1. 若JVM参数指定了catalina.config则根据值表示的路径加载配置文件，若文件存在则转到4，否则转到2；
  2. 尝试加载静态代码块解析的catalina.base的子目录conf中的catalina.properties文件，若文件存在则转到4，否则转到3；
  3. 尝试加载tomcat自身jar包中的org/apache/catalina/startup/catalina.properties文件；
  4. 将配置文件中的属性和值复制到系统属性中。
• 利用反射实例化Catalina类。

load函数

Bootstrap类的load函数代码如下，该函数利用反射在init函数生成的Catalina类实例上调用load函数，参数即为Bootstrap的命令行参数。

private void load(String[] arguments) throws Exception {
    // Call the load() method
    String methodName = "load";
    Object param[];
    Class<?> paramTypes[];
    if (arguments==null || arguments.length==0) {
        paramTypes = null;
        param = null;
    } else {
        paramTypes = new Class[1];
        paramTypes[0] = arguments.getClass();
        param = new Object[1];
        param[0] = arguments;
    }
    Method method =
        catalinaDaemon.getClass().getMethod(methodName, paramTypes);
    if (log.isDebugEnabled())
        log.debug("Calling startup class " + method);
    method.invoke(catalinaDaemon, param);
}

main函数

在一般的启动流程中，Bootstrap类的main函数会先执行init函数，然后执行load函数，最后在start函数中利用反射调用Catalina类实例的start方法。

public static void main(String args[]) {
    if (daemon == null) {
        // Don't set daemon until init() has completed
        Bootstrap bootstrap = new Bootstrap();
        try {
            bootstrap.init();
        } catch (Throwable t) {
            handleThrowable(t);
            t.printStackTrace();
            return;
        }
        daemon = bootstrap;
    } else {
        // When running as a service the call to stop will be on a new
        // thread so make sure the correct class loader is used to prevent
        // a range of class not found exceptions.
        Thread.currentThread().setContextClassLoader(daemon.catalinaLoader);
    }

    try {
        String command = "start";
        if (args.length > 0) {
            command = args[args.length - 1];
        }

        if (command.equals("startd")) {
            args[args.length - 1] = "start";
            daemon.load(args);
            daemon.start();
        } else if (command.equals("stopd")) {
            args[args.length - 1] = "stop";
            daemon.stop();
        } else if (command.equals("start")) {
            daemon.setAwait(true);
            daemon.load(args);
            daemon.start();
        } else if (command.equals("stop")) {
            daemon.stopServer(args);
        } else if (command.equals("configtest")) {
            daemon.load(args);
            if (null==daemon.getServer()) {
                System.exit(1);
            }
            System.exit(0);
        } else {
            log.warn("Bootstrap: command \"" + command + "\" does not exist.");
        }
    } catch (Throwable t) {
        // Unwrap the Exception for clearer error reporting
        if (t instanceof InvocationTargetException &&
                t.getCause() != null) {
            t = t.getCause();
        }
        handleThrowable(t);
        t.printStackTrace();
        System.exit(1);
    }

}

// 省略了一些代码
public void start()
    throws Exception {
    if( catalinaDaemon==null ) init();
    Method method = catalinaDaemon.getClass().getMethod("start", (Class [] )null);
    method.invoke(catalinaDaemon, (Object [])null);
}
// 省略了一些代码

接下来的重点是Catalina类。