Support for reading offset from a file

.gitignore

@@ -1,2 +1,4 @@
 out/
 *.class
+lib/
+test/

FakeTimeFileTest.java

@@ -0,0 +1,21 @@
+import java.util.Calendar;
+import java.lang.String;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+public class FakeTimeFileTest {
+  public static void main(String args[]) throws Throwable{
+    final Path filePath = Files.createTempFile(null, null);
+    System.out.println("Using temporary file " + filePath.toString());
+
+    System.setProperty("faketime.offset.file", filePath.toAbsolutePath().toString());
+
+    for(int i = 0; i < 10; i++) {
+      Files.write(filePath, String.valueOf(i * -86400).getBytes("UTF-8"));
+
+      System.out.println("Time: " + System.currentTimeMillis());
+      Calendar calendar = Calendar.getInstance();
+      System.out.println("Date: " + calendar.getTime());
+    }
+  }
+}

README.md

@@ -48,6 +48,8 @@ gcc -fPIC -shared -I $JAVA_HOME/include -I $JAVA_HOME/include/linux -m32 -Wall s
 
 ### Step 2: Use it:
 
+##### Option 1: Control offset with a Java system property
+
 Run your Java program (say, org.test.Main) with these agent-specific extra arguments (see [issue #3](https://github.com/arvindsv/faketime/issues/3)), like this:
 
 ```
@@ -63,5 +65,21 @@ In your Java code, you can set the property **faketime.offset.seconds** to the n
 ```
 System.setProperty("faketime.offset.seconds", "86400");
 ```
+
+OR
+
+##### Option 2: Control offset with an offset file
+
+You can set the property **faketime.offset.file** to point to a file containing the offset in *seconds* (ASCII-encoded). This way you can alter the time using methods external to the Java process, i.e. through a mounted Kubernetes ConfigMap.
+
+```
+java -agentpath:/path/to/the/library/you/got/above \
+  -Dfaketime.offset.file=/path/to/a/text/file/containing/a/number
+  -XX:+UnlockDiagnosticVMOptions \
+  -XX:DisableIntrinsic=_currentTimeMillis \
+  -XX:CompileCommand=exclude,java/lang/System.currentTimeMillis \
+  org.test.Main
+```
+
 
 That's it! Take a look at [FakeTimeTest.java](https://github.com/arvindsv/faketime/blob/master/FakeTimeTest.java) if you need to see some Java code which uses it.

src/FakeTimeAgent.c

@@ -18,6 +18,34 @@ static jvmtiEnv *jvmti = NULL;
 static GlobalAgentData *gdata;
 static call__jlong originalMethodCurrentTimeInMillis = NULL;
 
+// Read the offset in seconds from a text file and convert it to a long in milliseconds.
+long readOffsetFromFile(const char* filePath) {
+  FILE* pFile = fopen(filePath, "r");
+
+  if (pFile == NULL) {
+    return 0L;
+  }
+
+  // Find the size of the file
+  fseek(pFile, 0, SEEK_END);
+  long lSize = ftell(pFile);
+  rewind(pFile);
+
+  // Allocate a buffer for the file contents, adding room for NULL-termination.
+  char* buffer = (char*) malloc(sizeof(char) * (lSize + 1));
+  fread(buffer, sizeof(char), lSize, pFile);
+  fclose(pFile);
+
+  buffer[lSize] = '\0'; // Make sure the string is null terminated
+
+  // Convert the offset string to a long and multiply by 1000 to get milliseconds
+  long offset = atol(buffer) * 1000;
+
+  free(buffer);
+
+  return offset;
+}
+
 JNIEXPORT jlong JNICALL newCurrentTimeInMillis(JNIEnv* env, jclass jc) {
   jclass systemClass = (*env)->FindClass(env, "java/lang/System");
   jmethodID getPropertyMethodId = (*env)->GetStaticMethodID(env, systemClass, "getProperty", "(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;");
@@ -28,16 +56,31 @@ JNIEXPORT jlong JNICALL newCurrentTimeInMillis(JNIEnv* env, jclass jc) {
 
   jstring offsetPropertyName = (*env)->NewStringUTF(env, "faketime.offset.seconds");
   jstring offsetPropertyDefault = (*env)->NewStringUTF(env, "0");
+  jstring filePathPropertyName = (*env)->NewStringUTF(env, "faketime.offset.file");
+  //jstring fileCacheDurationPropertyName = (*env)->NewStringUTF(env, "faketime.file.cache_duration");
+  //jstring fileCacheDurationPropertyDefault = (*env)->NewStringUTF(env, "10");
 
   jstring offsetValue = (*env)->CallStaticObjectMethod(env, systemClass, getPropertyMethodId, offsetPropertyName, offsetPropertyDefault);
+  jstring filePath = (*env)->CallStaticObjectMethod(env, systemClass, getPropertyMethodId, filePathPropertyName, NULL);
+  //jstring fileCacheDuration = (*env)->CallStaticObjectMethod(env, systemClass, getPropertyMethodId, fileCacheDurationPropertyName, fileCacheDurationPropertyDefault);
   if ((*env)->ExceptionCheck(env)) return 0;
 
-  const char *offset = (*env)->GetStringUTFChars(env, offsetValue, NULL);
+  jlong offset;
+  if (filePath != NULL) {
+    // If filePath is defined, read the offset from that file (in milliseconds)
+    const char* filepathStr = (*env)->GetStringUTFChars(env, filePath, NULL);
+    offset = readOffsetFromFile(filepathStr);
+    (*env)->ReleaseStringUTFChars(env, filePath, filepathStr);
+  } else {
+    // if not, use the offset from system property "faketime.offset.seconds"
+    const char *offsetStr = (*env)->GetStringUTFChars(env, offsetValue, NULL);
+    offset = atol(offsetStr) * 1000;
+    (*env)->ReleaseStringUTFChars(env, offsetValue, offsetStr);
+  }
 
   jlong realTime = originalMethodCurrentTimeInMillis(env, jc);
-  jlong timeWithOffset = realTime + (atol(offset) * 1000);
+  jlong timeWithOffset = realTime + offset;
 
-  (*env)->ReleaseStringUTFChars(env, offsetValue, offset);
   return timeWithOffset;
 }
 

test_linux.sh

@@ -5,4 +5,7 @@ gcc -shared -I $JAVA_HOME/include -I $JAVA_HOME/include/linux -m32 -Wall src/Fak
 mkdir -p lib/linux && mv libfaketime.so lib/linux/ 
 
 javac FakeTimeTest.java 
+javac FakeTimeFileTest.java
+
 java -agentpath:./lib/linux/libfaketime.so FakeTimeTest
+java -agentpath:./libfaketime.so FakeTimeFileTest

test_linux_64.sh

@@ -2,7 +2,10 @@
 set -eu
 
 gcc -fPIC -shared -I $JAVA_HOME/include -I $JAVA_HOME/include/linux -m64 -Wall src/FakeTimeAgent.c -o libfaketime.so 
-mkdir -p lib/linux && mv libfaketime.so lib/linux/ 
+mkdir -p lib/linux && mv libfaketime.so lib/linux/
+
+javac FakeTimeTest.java
+javac FakeTimeFileTest.java
 
-javac FakeTimeTest.java 
 java -agentpath:./lib/linux/libfaketime.so FakeTimeTest
+java -agentpath:./lib/linux/libfaketime.so FakeTimeFileTest

test_mac.sh

@@ -5,4 +5,7 @@ gcc -shared -I $JAVA_HOME/include -Wall src/FakeTimeAgent.c -o libfaketime.jnili
 mkdir -p lib/mac/ && mv libfaketime.jnilib lib/mac/
 
 javac FakeTimeTest.java 
+javac FakeTimeFileTest.java
+
 java -agentpath:./lib/mac/libfaketime.jnilib FakeTimeTest
+java -agentpath:./lib/mac/libfaketime.jnilib FakeTimeFileTest