带你掌握不同平台下，探索JDK源码所需的native方法

 /** * Gets the platform defined TimeZone ID. **/ private static native String getSystemTimeZoneID(String javaHome);

getSystemTimeZoneID方法所在的package java.util.TimeZone;

/* * Gets the platform defined TimeZone ID */ JNIEXPORT jstring JNICALL Java_java_util_TimeZone_getSystemTimeZoneID(JNIEnv *env, jclass ign, jstring java_home, jstring country) { const char *cname; const char *java_home_dir; char *javaTZ; if (java_home == NULL) return NULL; java_home_dir = JNU_GetStringPlatformChars(env, java_home, 0); if (java_home_dir == NULL) return NULL; if (country != NULL) { cname = JNU_GetStringPlatformChars(env, country, 0); /* ignore error cases for cname */ } else { cname = NULL; } /* * Invoke platform dependent mapping function */ javaTZ = findJavaTZ_md(java_home_dir, cname); free((void *)java_home_dir); if (cname != NULL) { free((void *)cname); } if (javaTZ != NULL) { jstring jstrJavaTZ = JNU_NewStringPlatform(env, javaTZ); free((void *)javaTZ); return jstrJavaTZ; } return NULL; }

 /* * Invoke platform dependent mapping function */ javaTZ = findJavaTZ_md(java_home_dir, cname);

因为OpenJDK里，Java标准库和部分工具的源码repo（jdk目录）里，BSD和Linux的平台相关源码都是在solaris目录里的。 原本Sun JDK的源码里平台相关的目录就是从solaris和windows这两个目录开始的，后来Unix系的平台相关代码全都放在solaris目录下了，共用大部分代码。 作者：RednaxelaFX 链接：https://www.zhihu.com/question/58982441/answer/170264788 来源：知乎

/* * Detects the platform time zone which maps to a Java time zone ID. */ char *findJavaTZ_md(const char *java_home_dir, const char *country) { char winZoneName[MAX_ZONE_CHAR]; char winMapID[MAX_MAPID_LENGTH]; char *std_timezOne= NULL; int result; winMapID[0] = 0; result = getWinTimeZone(winZoneName, winMapID); if (result != VALUE_UNKNOWN) { if (result == VALUE_GMTOFFSET) { std_timezOne= _strdup(winZoneName); } else { std_timezOne= matchJavaTZ(java_home_dir, result, winZoneName, winMapID, country); } } return std_timezone; }

/* * Gets the current time zone entry in the "Time Zones" registry. */ static int getWinTimeZone(char *winZoneName, char *winMapID) { ... }

计算机\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\

/* * findJavaTZ_md() maps platform time zone ID to Java time zone ID * using /lib/tzmappings. If the TZ value is not found, it * trys some libc implementation dependent mappings. If it still * can't map to a Java time zone ID, it falls back to the GMT+/-hh:mm * form. `country', which can be null, is not used for UNIX platforms. */ /*ARGSUSED1*/ char * findJavaTZ_md(const char *java_home_dir, const char *country) { char *tz; char *javatz = NULL; char *freetz = NULL; tz = getenv("TZ"); #ifdef __linux__ if (tz == NULL) { #else #ifdef __solaris__ if (tz == NULL || *tz == '\0') { #endif #endif tz = getPlatformTimeZoneID(); freetz = tz; } /* * Remove any preceding ':' */ if (tz != NULL && *tz == ':') { tz++; } #ifdef __solaris__ if (strcmp(tz, "localtime") == 0) { tz = getSolarisDefaultZoneID(); freetz = tz; } #endif if (tz != NULL) { #ifdef __linux__ /* * Ignore "posix/" prefix. */ if (strncmp(tz, "posix/", 6) == 0) { tz += 6; } #endif javatz = strdup(tz); if (freetz != NULL) { free((void *) freetz); } } return javatz; } 

timedatectl

timedatectl set-timezone Asia/Shanghai