原文地址： https://caoxudong818.iteye.com/blog/1565980
以下内容均已jdk1.7.0_04为准。
啥是 Serviceability ？
Serviceability Agent (SA) The Serviceablity Agent is collection of Sun internal code that aids in debugging HotSpot problems. It is also used by several JDK tools - jstack, jmap, jinfo, and jdb. See SA for more information.
简单说，这部分是用来调试查看hotspot的。
子曰，学而时习之 不亦说乎。这里就先习之，有个直观印象之后，再学之。 平时用来查看hotspot内部信息的常用的工具都在$JAVA_HOME/bin目录下，其中一些工具就是用Serviceability开发的。这里要介绍的是HSDB（其命令版本是CLHSDB），启动方式如下：
java -classpath $JAVA_HOME/lib/sa-jdi.jar sun.jvm.hotspot.HSDB
看到界面后，大家自己琢磨怎么玩吧，内容挺多。
类似的，启动CLHSDB的方式如下：
java -classpath $JAVA_HOME/lib/sa-jdi.jar sun.jvm.hotspot.CLHSDB
这里有一个简单的说明文档：https://hg.openjdk.java.net/hsx/hotspot-main/hotspot/raw-file/tip/agent/doc/clhsdb.html
另外，还有一个DebugServer类，功能差不多。
java -classpath %JAVA_HOME%/lib/sa-jdi.jar sun.jvm.hotspot.DebugServer
在sun.jvm.hotspot包下，有个HelloWorld类。我觉着此类是在蛋疼的紧，各位请看代码：
/*
* Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/
package sun.jvm.hotspot;
import java.lang.reflect.*;
public class HelloWorld {
private static String helloWorldString = "Hello, world!";
private static volatile int helloWorldTrigger = 0;
private static final boolean useMethodInvoke = false;
private static Object lock = new Object();
public static void main(String[] args) {
int foo = a();
System.out.println("HelloWorld exiting. a() = " + foo);
}
private static int a() {
return 1 + b();
}
private static int b() {
return 1 + c();
}
private static int c() {
return 1 + d("Hi");
}
private static int d(String x) {
System.out.println("HelloWorld.d() received \"" + x + "\" as argument");
synchronized(lock) {
if (useMethodInvoke) {
try {
Method method = HelloWorld.class.getMethod("e");
Integer result = (Integer) method.invoke(null, new Object[0]);
return result.intValue();
}
catch (Exception e) {
throw new RuntimeException(e.toString());
}
} else {
int i = fib(10); // 89
long l = i;
float f = i;
double d = i;
char c = (char) i;
short s = (short) i;
byte b = (byte) i;
int ret = e();
System.out.println("Tenth Fibonacci number in all formats: " +
i + ", " +
l + ", " +
f + ", " +
d + ", " +
c + ", " +
s + ", " +
b);
return ret;
}
}
}
public static int e() {
System.out.println("Going to sleep...");
int i = 0;
while (helloWorldTrigger == 0) {
if (++i == 1000000) {
System.gc();
}
}
System.out.println(helloWorldString);
while (helloWorldTrigger != 0) {}
return i;
}
// Tree-recursive implementation for test
public static int fib(int n) {
if (n < 2) {
return 1;
}
return fib(n - 1) + fib(n - 2);
}
}
难道此类是用来硬件测性能的？
sa中的包主要分为以下几个部分： asm，ci，code，debugger，gc，interpreter，jdi，livevm，memory，oops，opto，prims，runtine，tools，types。 后续的文章会进行介绍，可能会跳过一些（我不懂的）。
回到CLHSDB，这个只是一个壳，主要的功能都是靠sun.jvm.hotspot.HotSpotAgent和sun.jvm.hotspot.CommandProcessor完成的。
CommandProcessor.DebuggerInterface di = new CommandProcessor.DebuggerInterface() {
public HotSpotAgent getAgent() {
return agent;
}
public boolean isAttached() {
return attached;
}
public void attach(String pid) {
attachDebugger(pid);
}
public void attach(String java, String core) {
attachDebugger(java, core);
}
public void detach() {
detachDebugger();
}
public void reattach() {
if (attached) {
detachDebugger();
}
if (pidText != null) {
attach(pidText);
} else {
attach(execPath, coreFilename);
}
}
};
BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
CommandProcessor cp = new CommandProcessor(di, in, System.out, System.err);</pre>
构造CommandProcessor类的实例前时，会对其中的一个成员变量commandList进行初始化，其中存储的就是预先注册的可执行命令：
private final Command[] commandList = {
new Command("reattach", true) {
public void doit(Tokens t) {
int tokens = t.countTokens();
if (tokens != 0) {
usage();
return;
}
preAttach();
debugger.reattach();
postAttach();
}
},
new Command("attach", "attach pid | exec core", true) {
public void doit(Tokens t) {
int tokens = t.countTokens();
if (tokens == 1) {
preAttach();
debugger.attach(t.nextToken());
postAttach();
} else if (tokens == 2) {
preAttach();
debugger.attach(t.nextToken(), t.nextToken());
postAttach();
} else {
usage();
}
}
},
......
new Command("assert", "assert true | false", true) {
public void doit(Tokens t) {
if (t.countTokens() != 1) {
usage();
} else {
Assert.ASSERTS_ENABLED = Boolean.valueOf(t.nextToken()).booleanValue();
}
}
},
};
这样，程序就可以接收并处理用户输入的命令。
HotSpotAgent类的主要功能是根据主机环境设置调试器（debugger），并将debugger attach（这词儿该咋表述）到目标上（目标可以通过PID或dump文件）。流程如下（以Windows为例）： attach($pid) -> setupDebugger() -> setupDebuggerWin32() -> attachDebugger() 在这期间会根据需要，判断是否建立一个远程调式服务器，以便于执行远程调试。
if (isServer) {
RemoteDebuggerServer remote = null;
try {
remote = new RemoteDebuggerServer(debugger);
}catch (RemoteException rem) {
throw new DebuggerException(rem);
}
RMIHelper.rebind(serverID, remote);
}
to be continued…