I need 开发者_Python百科to work around a Java bug in JDK 1.5 which was fixed in 1.6. I'm using the following condition:

if (System.getProperty("java.version").startsWith("1.5.")) {
    ...
} else {
    ...
}

Will this work for other JVMs? Is there a better way to check this?

java.version is a system property that exists in every JVM. There are two possible formats for it:

• Java 8 or lower: 1.6.0_23, 1.7.0, 1.7.0_80, 1.8.0_211
• Java 9 or higher: 9.0.1, 11.0.4, 12, 12.0.1

Here is a trick to extract the major version: If it is a 1.x.y_z version string, extract the character at index 2 of the string. If it is a x.y.z version string, cut the string to its first dot character, if one exists.

private static int getVersion() {
    String version = System.getProperty("java.version");
    if(version.startsWith("1.")) {
        version = version.substring(2, 3);
    } else {
        int dot = version.indexOf(".");
        if(dot != -1) { version = version.substring(0, dot); }
    } return Integer.parseInt(version);
}

Now you can check the version much more comfortably:

if(getVersion() < 6) {
    // ...
}

What about getting the version from the package meta infos:

String version = Runtime.class.getPackage().getImplementationVersion();

Prints out something like:

1.7.0_13

Runtime.version()

Since Java 9, you can use Runtime.version(), which returns a Runtime.Version:

Runtime.Version version = Runtime.version();

These articles seem to suggest that checking for 1.5 or 1.6 prefix should work, as it follows proper version naming convention.

Sun Technical Articles

• J2SE SDK/JRE Version String Naming Convention
• Version 1.5.0 or 5.0?
  • "J2SE also keeps the version number 1.5.0 (or 1.5) in some places that are visible only to developers, or where the version number is parsed by programs"
    • "java.version system property"
• Version 1.6.0 Used by Developers
  • "Java SE keeps the version number 1.6.0 (or 1.6) in some places that are visible only to developers, or where the version number is parsed by programs."
    • "java.version system property"

The simplest way (java.specification.version):

double version = Double.parseDouble(System.getProperty("java.specification.version"));

if (version == 1.5) {
    // 1.5 specific code
} else {
    // ...
}

or something like (java.version):

String[] javaVersionElements = System.getProperty("java.version").split("\\.");

int major = Integer.parseInt(javaVersionElements[1]);

if (major == 5) {
    // 1.5 specific code
} else {
    // ...
}

or if you want to break it all up (java.runtime.version):

String discard, major, minor, update, build;

String[] javaVersionElements = System.getProperty("java.runtime.version").split("\\.|_|-b");

discard = javaVersionElements[0];
major   = javaVersionElements[1];
minor   = javaVersionElements[2];
update  = javaVersionElements[3];
build   = javaVersionElements[4];

Example for Apache Commons Lang:

import org.apache.commons.lang.SystemUtils;

    Float version = SystemUtils.JAVA_VERSION_FLOAT;

    if (version < 1.4f) { 
        // legacy
    } else if (SystemUtils.IS_JAVA_1_5) {
        // 1.5 specific code
    } else if (SystemUtils.isJavaVersionAtLeast(1.6f)) {
        // 1.6 compatible code
    } else {
        // dodgy clause to catch 1.4 :)
    }

Just a note that in Java 9 and above, the naming convention is different. System.getProperty("java.version") returns "9" rather than "1.9".

Does not work, need --pos to evaluate double:

    String version = System.getProperty("java.version");
    System.out.println("version:" + version);
    int pos = 0, count = 0;
    for (; pos < version.length() && count < 2; pos++) {
        if (version.charAt(pos) == '.') {
            count++;
        }
    }

    --pos; //EVALUATE double

    double dversion = Double.parseDouble(version.substring(0, pos));
    System.out.println("dversion:" + dversion);
    return dversion;
}

Here's the implementation in JOSM:

/**
 * Returns the Java version as an int value.
 * @return the Java version as an int value (8, 9, etc.)
 * @since 12130
 */
public static int getJavaVersion() {
    String version = System.getProperty("java.version");
    if (version.startsWith("1.")) {
        version = version.substring(2);
    }
    // Allow these formats:
    // 1.8.0_72-ea
    // 9-ea
    // 9
    // 9.0.1
    int dotPos = version.indexOf('.');
    int dashPos = version.indexOf('-');
    return Integer.parseInt(version.substring(0,
            dotPos > -1 ? dotPos : dashPos > -1 ? dashPos : 1));
}

If you can have dependency to apache utils you can use org.apache.commons.lang3.SystemUtils.

    System.out.println("Is Java version at least 1.8: " + SystemUtils.isJavaVersionAtLeast(JavaVersion.JAVA_1_8));

Don't know another way of checking this, but this: http://java.sun.com/j2se/1.5.0/docs/api/java/lang/System.html#getProperties()" implies "java.version" is a standard system property so I'd expect it to work with other JVMs.

Here is the answer from @mvanle, converted to Scala: scala> val Array(javaVerPrefix, javaVerMajor, javaVerMinor, _, _) = System.getProperty("java.runtime.version").split("\\.|_|-b") javaVerPrefix: String = 1 javaVerMajor: String = 8 javaVerMinor: String = 0

https://docs.oracle.com/javase/9/docs/api/java/lang/Runtime.Version.html#version--

 Runtime.version().version()

For 17.0.1 it returns [17, 0, 1] One can use Runtime.version().version().get(0) to get the major java version.

In kotlin:

/**
 * Returns the major JVM version, e.g. 6 for Java 1.6, 8 for Java 8, 11 for Java 11 etc.
 */
public val jvmVersion: Int get() = System.getProperty("java.version").parseJvmVersion()

/**
 * Returns the major JVM version, 1 for 1.1, 2 for 1.2, 3 for 1.3, 4 for 1.4, 5
 * for 1.5 etc.
 */
fun String.parseJvmVersion(): Int {
    val version: String = removePrefix("1.").takeWhile { it.isDigit() }
    return version.toInt()
}