如何在Java 9上运行jol?

Question

我正在尝试使用Java 9的jol运行一个程序，但没有运气。

我在pom.xml有以下依赖关系：

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

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 中的 VM 参数中添加了-Djdk.attach.allowAttachSelf=true，但它没有帮助(仍然是相同的输出)。

附言 我可以从类路径成功运行程序。尽管如此，如何从模块路径运行它仍然很有趣。

Answer 1

好吧，我尝试进一步调试它，发现警告的原因InstrumentationSupport尝试在应用程序启动时DYNAMIC_ATTACH。代码实际使用VirtualMachine的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);
...

该类未加载初始解析的模块，因为它的包是从模块描述符中当前缺少jdk.attach模块导出的。因此，您可以更新为使用模块声明为：

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

然后允许使用 VM 选项进一步自连接，如下所示：-

-Djdk.attach.allowAttachSelf=true

---

在执行共享代码 [VM.current().details()] 时，输出应包含详细信息，如 -

/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