Sometimes you might need to establish beyond doubt what version of Java a class has been compiled to. You need to examine the header to find that out. The below shows how to do that. The source code is HERE. I've since updated the source code to cope with more modern versions of Java class files.
import java.io.*;
public class JavaVersion {
private final static int HEADER_LENGTH = 8;
public static void main(String[] args) {
if (args.length < 1) {
System.err.println("Usage: java JavaVersion <class file>");
System.exit(1);
}
DataInputStream in = null;
try {
File f = new File(args[0]);
in = new DataInputStream(new FileInputStream(f));
byte[] header = new byte[HEADER_LENGTH];
in.readFully(header);
int javaVersion = header[HEADER_LENGTH - 1] & 0xFF;
String versionString = null;
switch (javaVersion) {
case 0x34:
versionString = "1.8";
break;
case 0x33:
versionString = "1.7";
break;
case 0x32:
versionString = "1.6";
break;
case 0x31:
versionString = "1.5";
break;
case 0x30:
versionString = "1.4";
break;
case 0x2F:
versionString = "1.3";
break;
case 0x2E:
versionString = "1.2";
break;
default:
versionString = "UNKNOWN_VERSION";
}
System.out.println("Java class file version is " + versionString);
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
in.close();
} catch (Exception e) { /* ignore */
}
}
}
}
public class JavaVersion {
private final static int HEADER_LENGTH = 8;
public static void main(String[] args) {
if (args.length < 1) {
System.err.println("Usage: java JavaVersion <class file>");
System.exit(1);
}
DataInputStream in = null;
try {
File f = new File(args[0]);
in = new DataInputStream(new FileInputStream(f));
byte[] header = new byte[HEADER_LENGTH];
in.readFully(header);
int javaVersion = header[HEADER_LENGTH - 1] & 0xFF;
String versionString = null;
switch (javaVersion) {
case 0x34:
versionString = "1.8";
break;
case 0x33:
versionString = "1.7";
break;
case 0x32:
versionString = "1.6";
break;
case 0x31:
versionString = "1.5";
break;
case 0x30:
versionString = "1.4";
break;
case 0x2F:
versionString = "1.3";
break;
case 0x2E:
versionString = "1.2";
break;
default:
versionString = "UNKNOWN_VERSION";
}
System.out.println("Java class file version is " + versionString);
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
in.close();
} catch (Exception e) { /* ignore */
}
}
}
}
