JVM Flags access API #149
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Introduce an API to access JVM flags directly
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,157 @@ | ||
| package com.datadoghq.profiler; | ||
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| import java.util.function.Consumer; | ||
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| /** | ||
| * An internal JVM access support. | ||
| * <p> | ||
| * We are using vmstructs and dynamic symbol lookups to provide access to some JVM internals. | ||
| * There will be dragons here. We are touching and possibly mutating JVM internals. Do not use | ||
| * unless you know what you are doing. | ||
| * </p> | ||
| */ | ||
| public final class JVMAccess { | ||
| private static final class SingletonHolder { | ||
| static final JVMAccess INSTANCE = new JVMAccess(); | ||
| } | ||
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| /** | ||
| * Flags interface to access JVM flags. | ||
| * In general, the flags are read-only. However, some flags can be modified at runtime. | ||
| * Currently, only string and boolean flags can be modified. Allowing modification of numeric | ||
| * flags would require exact specification of the flag type (int, long, float, double) such | ||
| * that the correct number of bytes would be written to the flag and not overwrite the surrounding | ||
| * memory. | ||
| */ | ||
| public interface Flags { | ||
| Flags NONE = new Flags() { | ||
| @Override | ||
| public String getStringFlag(String name) { | ||
| return null; | ||
| } | ||
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| @Override | ||
| public void setStringFlag(String name, String value) { | ||
| } | ||
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| @Override | ||
| public boolean getBooleanFlag(String name) { | ||
| return false; | ||
| } | ||
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| @Override | ||
| public void setBooleanFlag(String name, boolean value) { | ||
| } | ||
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| @Override | ||
| public long getIntFlag(String name) { | ||
| return 0; | ||
| } | ||
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| @Override | ||
| public double getFloatFlag(String name) { | ||
| return 0; | ||
| } | ||
| }; | ||
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| String getStringFlag(String name); | ||
| void setStringFlag(String name, String value); | ||
| boolean getBooleanFlag(String name); | ||
| void setBooleanFlag(String name, boolean value); | ||
| long getIntFlag(String name); | ||
| double getFloatFlag(String name); | ||
| } | ||
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| private class FlagsImpl implements Flags { | ||
| public String getStringFlag(String name) { | ||
| return findStringJVMFlag0(name); | ||
| } | ||
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| public void setStringFlag(String name, String value) { | ||
| setStringJVMFlag0(name, value); | ||
| } | ||
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| public boolean getBooleanFlag(String name) { | ||
|
jbachorik marked this conversation as resolved.
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| return findBooleanJVMFlag0(name); | ||
| } | ||
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| public void setBooleanFlag(String name, boolean value) { | ||
| setBooleanJVMFlag0(name, value); | ||
| } | ||
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| public long getIntFlag(String name) { | ||
| return findIntJVMFlag0(name); | ||
| } | ||
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| public double getFloatFlag(String name) { | ||
| return findFloatJVMFlag0(name); | ||
| } | ||
| } | ||
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| public static JVMAccess getInstance() { | ||
| return SingletonHolder.INSTANCE; | ||
| } | ||
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| private final LibraryLoader.Result libraryLoadResult; | ||
| private final Flags flags; | ||
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| private JVMAccess() { | ||
| LibraryLoader.Result result = LibraryLoader.builder().load();; | ||
| if (result.success) { | ||
| // library loaded successfully, check if we can access JVM | ||
| try { | ||
| healthCheck0(); | ||
| } catch (Throwable t) { | ||
| // failed to access JVM; update the result | ||
| result = new LibraryLoader.Result(false, t); | ||
| } | ||
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| } | ||
| if (!result.success && result.error != null) { | ||
| System.out.println("[WARNING] Failed to obtain JVM access.\n" + result.error); | ||
|
MattAlp marked this conversation as resolved.
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| } | ||
| flags = result.success ? new FlagsImpl() : Flags.NONE; | ||
| libraryLoadResult = result; | ||
| } | ||
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| public JVMAccess(String libLocation, String scratchDir, Consumer<Throwable> errorHandler) { | ||
| LibraryLoader.Result result = LibraryLoader.builder().withLibraryLocation(libLocation).withScratchDir(scratchDir).load(); | ||
| if (result.success) { | ||
| // library loaded successfully, check if we can access JVM | ||
| try { | ||
| healthCheck0(); | ||
| } catch (Throwable t) { | ||
| // failed to access JVM; update the result | ||
| result = new LibraryLoader.Result(false, t); | ||
| } | ||
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| } | ||
| if (!result.success && result.error != null) { | ||
| if (errorHandler != null) { | ||
| errorHandler.accept(result.error); | ||
| } else { | ||
| System.out.println("[WARNING] Failed to obtain JVM access.\n" + result.error); | ||
|
MattAlp marked this conversation as resolved.
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| } | ||
| } | ||
| flags = result.success ? new FlagsImpl() : Flags.NONE; | ||
| libraryLoadResult = result; | ||
| } | ||
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| public Flags flags() { | ||
| return flags; | ||
| } | ||
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| public boolean isActive() { | ||
| return libraryLoadResult.success; | ||
| } | ||
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| // a dummy method to check if the library has loaded properly | ||
| private native boolean healthCheck0(); | ||
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| private native String findStringJVMFlag0(String name); | ||
| private native void setStringJVMFlag0(String name, String value); | ||
| private native boolean findBooleanJVMFlag0(String name); | ||
| private native void setBooleanJVMFlag0(String name, boolean value); | ||
| private native long findIntJVMFlag0(String name); | ||
| private native double findFloatJVMFlag0(String name); | ||
| } | ||
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I would specify as-of details here
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Sorry, I don't understand - what should I specify here?