Since Java 11, a PREVIEW-MODE on the Java Compiler and Runtime can be enabled. It allows to preview new features. (JEP 12)
java --enable-preview
How can I detect from within Java whether the JVM has been started with preview-mode enabled?
The intention is to describe the running container on an in-application status page/json - for devops overview.
So far I looked into system-properties and flags, neither contained a hint.
You can check the Java Class File minor_version. If 0xFFFF the class file is compiled with --enable-preview. For details, see https://stackoverflow.com/a/58821511/868941.
A simple program to check this is given below (beware, Java 13 code with preview features enabled!).
public final class ClassFileVersion {
private final int major;
private final int minor;
private ClassFileVersion(int major, int minor) {
this.major = major;
this.minor = minor;
}
public static ClassFileVersion of(Class<?> classFile) throws IOException {
try (InputStream is = classFile.getResourceAsStream("/%s.class".formatted(classFile.getName().replace('.', '/')))) {
var buffer = new byte[8];
if (is.read(buffer) != buffer.length) {
throw new AssertionError("Not a Java Class File!");
}
return new ClassFileVersion(readUnsignedShort(buffer, 6), readUnsignedShort(buffer, 4));
}
}
public String getVersionNumber() {
return "%d.%d".formatted(major, minor);
}
public boolean isEnablePreview() {
return major >= 55 && minor == 0xFFFF;
}
@Override
public String toString() {
return (major < 49 ? "JDK " : "Java SE ") +
switch(major) {
case 45 -> "1.1";
case 46 -> "1.2";
case 47 -> "1.3";
case 48 -> "1.4";
case 49 -> "5";
case 50 -> "6";
case 51 -> "7";
case 52 -> "8";
case 53 -> "9";
case 54 -> "10";
case 55 -> "11";
case 56 -> "12";
case 57 -> "13";
case 58 -> "14";
default -> throw wrongVersion();
} +
switch(minor) {
case 0 -> "";
case 3 -> {
if (major != 45) {
throw wrongVersion();
}
yield "";
}
case 0xFFFF -> " --enable-preview";
default -> throw wrongVersion();
};
}
private static int readUnsignedShort(byte[] buffer, int offset) {
return ((buffer[offset] & 0xff) << 8) + (buffer[++offset] & 0xff);
}
private AssertionError wrongVersion() {
return new AssertionError("Wrong Java Class File Version: %d.%d".formatted(major, minor));
}
// to run this code (JDK 13 needed):
// java --enable-preview --source 13 ClassFileVersion.java
public static void main(String[] args) throws IOException {
// prints "Java SE 13 --enable-preview"
System.out.println(ClassFileVersion.of(ClassFileVersion.class));
}
}