如何在Java 9上运行jol？

Question

我正试图使用jol和Java9一起运行一个程序，但没有成功。

我在pom.xml中有下面的依赖项

<dependency>
    <groupId>org.openjdk.jol</groupId>
    <artifactId>jol-core</artifactId>
    <version>0.9</version>
</dependency>

这个程序很简单：

package org.example;

import org.openjdk.jol.vm.VM;

public class Example {
    public static void main(String[] args) throws Throwable {
        System.out.println(VM.current().details());
    }
}

模块描述符：

module java9 {
    requires jol.core;
}

当我从IDEA运行程序时，我看到以下输出：

# WARNING: Unable to get Instrumentation. Dynamic Attach failed.
You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf

我在IDEA中将-Djdk.attach.allowAttachSelf=true添加到VM参数中，但是它没有帮助(仍然是相同的输出)。

我可以从类路径成功地运行程序。不过，如何从模块路径运行它还是很有趣的。

Answer 1

好的，我试过调试这一点--我发现警告的原因是InstrumentationSupport在应用程序启动时尝试了DYNAMIC_ATTACH。代码实际使用dynamicAttach的相关部分是

String name = "com.sun.tools.attach.VirtualMachine";
try {
    // JDK 9+ makes this class available on class path
    vmClass = ClassLoader.getSystemClassLoader().loadClass(name);
...

类没有加载最初解析的模块，因为它的包是从jdk.attach模块导出的，目前模块描述符中缺少该模块。因此，您可以将模块声明更新为：

module java9 {    // module name 'joltrial' in output
    requires jol.core;
    requires jdk.attach;
} 

然后使用VM选项允许自我附加，如：-

-Djdk.attach.allowAttachSelf=true

在执行共享代码VM.current().details()时，输出应包括以下细节：

/Library/Java/JavaVirtualMachines/jdk-9.0.1.jdk/Contents/Home/bin/java -Djdk.attach.allowAttachSelf=true "-javaagent:/Applications/IntelliJ IDEA 2017.3 CE EAP.app/Contents/lib/idea_rt.jar=53783:/Applications/IntelliJ IDEA 2017.3 CE EAP.app/Contents/bin" -Dfile.encoding=UTF-8 -p .../joltrial/target/classes:.../.m2/repository/org/openjdk/jol/jol-core/0.9/jol-core-0.9.jar -m joltrial/com.Sample
# WARNING: Unable to attach Serviceability Agent. Unable to attach even with module exceptions: [org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed.]
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# WARNING | Compressed references base/shifts are guessed by the experiment!
# WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
# WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]

Process finished with exit code 0