Java 工具（jmap,jstack）在Linux上的源碼分析（二）信號處理

當java虛擬機啟動的時候，會啟動很多內部的線程，這些線程主要在thread.cpp裡的create_vm方法體裡實現

而在thread.cpp裡主要起了2個線程來處理信號相關的

1. JvmtiExport::enter_live_phase();
3. // Signal Dispatcher needs to be started before VMInit event is posted
4. os::signal_init();
6. // Start Attach Listener if +StartAttachListener or it can't be started lazily
7. if (!DisableAttachMechanism) {
8. if (StartAttachListener || AttachListener::init_at_startup()) {
9. AttachListener::init();
10. }
11. }

1. Signal Dispatcher 線程

在os.cpp中的signal_init()函數中，啟動了signal dispatcher 線程，對signal dispather 線程主要是用於處理信號，等待信號並且分發處理，可以詳細看signal_thread_entry的方法

1. static void signal_thread_entry(JavaThread* thread, TRAPS) {
2. os::set_priority(thread, NearMaxPriority);
3. while (true) {
4. int sig;
5. {
6. // FIXME : Currently we have not decieded what should be the status
7. // for this java thread blocked here. Once we decide about
8. // that we should fix this.
9. sig = os::signal_wait();
10. }
11. if (sig == os::sigexitnum_pd()) {
12. // Terminate the signal thread
13. return;
14. }
16. switch (sig) {
17. case SIGBREAK: {
18. // Check if the signal is a trigger to start the Attach Listener - in that
19. // case don't print stack traces.
20. if (!DisableAttachMechanism && AttachListener::is_init_trigger()) {
21. continue;
22. }
23. // Print stack traces
24. // Any SIGBREAK operations added here should make sure to flush
25. // the output stream (e.g. tty->flush()) after output. See 4803766.
26. // Each module also prints an extra carriage return after its output.
27. VM_PrintThreads op;
28. VMThread::execute(&op);
29. VM_PrintJNI jni_op;
30. VMThread::execute(&jni_op);
31. VM_FindDeadlocks op1(tty);
32. VMThread::execute(&op1);
33. Universe::print_heap_at_SIGBREAK();
34. if (PrintClassHistogram) {
35. VM_GC_HeapInspection op1(gclog_or_tty, true /* force full GC before heap inspection */,
36. true /* need_prologue */);
37. VMThread::execute(&op1);
38. }
39. if (JvmtiExport::should_post_data_dump()) {
40. JvmtiExport::post_data_dump();
41. }
42. break;
43. }
44. default: {
45. // Dispatch the signal to java
46. HandleMark hm(THREAD);
47. klassOop k = SystemDictionary::resolve_or_null(vmSymbolHandles::sun_misc_Signal(), THREAD);
48. KlassHandle klass (THREAD, k);
49. if (klass.not_null()) {
50. JavaValue result(T_VOID);
51. JavaCallArguments args;
52. args.push_int(sig);
53. JavaCalls::call_static(
54. &result,
55. klass,
56. vmSymbolHandles::dispatch_name(),
57. vmSymbolHandles::int_void_signature(),
58. &args,
59. THREAD
60. );
61. }
62. if (HAS_PENDING_EXCEPTION) {
63. // tty is initialized early so we don't expect it to be null, but
64. // if it is we can't risk doing an initialization that might
65. // trigger additional out-of-memory conditions
66. if (tty != NULL) {
67. char klass_name[256];
68. char tmp_sig_name[16];
69. const char* sig_name = "UNKNOWN";
70. instanceKlass::cast(PENDING_EXCEPTION->klass())->
71. name()->as_klass_external_name(klass_name, 256);
72. if (os::exception_name(sig, tmp_sig_name, 16) != NULL)
73. sig_name = tmp_sig_name;
74. warning("Exception %s occurred dispatching signal %s to handler"
75. "- the VM may need to be forcibly terminated",
76. klass_name, sig_name );
77. }
78. CLEAR_PENDING_EXCEPTION;
79. }
80. }
81. }
82. }
83. }

可以看到通過os::signal_wait();等待信號，而在linux裡是通過sem_wait()來實現，接受到SIGBREAK(linux 中的QUIT)信號的時候（關於信號處理請參考筆者的另一篇:Java 中關於信號的處理在Linux下的實現 http://www.linuxidc.com/Linux/2012-01/51212.htm），第一次通過調用 AttachListener::is_init_trigger（）初始化attach listener線程，詳細見2.Attach Listener 線程。

1. 第一次收到信號，會開始初始化，當初始化成功，將會直接返回，而且不返回任何線程stack的信息（通過socket file的操作返回），並且第二次將不在需要初始化。如果初始化不成功，將直接在控制台的outputstream中打印線程棧信息。
2. 第二次收到信號，如果已經初始化過，將直接在控制台中打印線程的棧信息。如果沒有初始化，繼續初始化，走和第一次相同的流程。