Configuring the launch of the remote virtual machine to debug

Options need to be added to the standard launch of a virtual machine (VM) to enable the debugging architecture, allowing us to attach (hook in) and collect data.

-Xdebug	Enables remote debugging.
-Xnoagent	On a Sun VM, disables the proprietary debugging interface thus letting JPDA work correctly.在Sun VM上，禁用专有调试接口，从而允许JPDA正确工作。
-Djava.compiler=NONE	Disables Just-In-Time (JIT) compilation.. 我们一般debug程序的时候，只是关注其中的一部分代码，而且大部分情况下是设置断点，然后单步执行，而JIT的编译单位是class，只要我们执行了class里面的代码，JIT就会对整个class进行编译，而我们实际执行的代码一般都是其中的一部分代码，所以从整个时间效率上来看，采用JIT反而更费时间。也就是说在JVM远程调试这个事情上，禁用JIT（只使用转译器，解释一行执行一条）更合理，所以通过-Djava.compiler=NONE来禁止JIT。
-Xrunjdwp: transport=dt_socket, server=y, address=5000, suspend=y	Loads JDWP (Java Debug Wire Protocol), the reference implementation of JPDA. Sub-options further specify the option.
transport=dt_socket	Sets the transport type for the connection with the debugging application; in this example, we will connect via a socket.
server=y	Tells the VM whether it must be receptive to an attaching debugging application.
address=5000	The port address for our connection; we will need 5000 for our example.
suspend=y	This option can be set depending on whether we want to suspend the exectution of the VM until a debugging application connects. This is useful if we seek to understand what happens when a server starts.

1、If we wish to make the VM wait for a connection before a full launch, we will need a command line such as this :

java …. -Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,server=y,address=5000,suspend=y

2、If we want the VM to be fully executing and listening for a debugging application, we will require a command line such as this one :

java …. -Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,server=y,address=5000,suspend=n

 第2种模式是一种比较好的方式，VM采用侦听的方式等待调试程序的连接。