Understanding -XX:+PrintCompilation Output in Java

The -XX:+PrintCompilation flag in the Java Virtual Machine (JVM) prints information about the methods being compiled by the Just-In-Time (JIT) compiler. When you enable this flag, the JVM will output a log of compilation events to the standard output. Each line of the output provides information about a specific method being compiled. Here, I'll explain the meaning of the different columns and markers, specifically focusing on the n, s, and % markers as seen in your example.

Explanation of Output Columns and Markers

Here's a breakdown of what each column and marker means:

• Timestamp: The time (in milliseconds) since the JVM started when the compilation event occurred.
• Compilation ID: A unique identifier for each compilation task within the JVM's lifecycle.
• Optimization Level: The level of optimization applied (e.g., 1, 3, 4).
• Method Name and Details: The fully qualified name of the method being compiled, along with the size of the method in bytes.
• Marker: Indicates special conditions or additional information about the compilation.

Common Markers

• n: Indicates that the method is a native method.
• s: Indicates that the method is synchronized.
• %: Indicates that the method has been compiled at a higher optimization level.

Detailed Breakdown of Example Output

Let's go through a portion of your example output line by line:

44    1       3       java.lang.String::charAt (29 bytes)
46    2     n 0       java.lang.System::arraycopy (native)   (static)
48    3       3       java.lang.String::hashCode (55 bytes)
48    4       3       java.lang.String::indexOf (70 bytes)
49    5       3       java.lang.String::equals (81 bytes)
49    7       3       java.lang.Object:: (1 bytes)
49    6       3       java.lang.String::length (6 bytes)
51    8       3       java.lang.AbstractStringBuilder::ensureCapacityInternal (27 bytes)
52    9       3       java.lang.CharacterData::of (120 bytes)
53   14       3       java.lang.AbstractStringBuilder::append (29 bytes)
53   10       3       java.lang.CharacterDataLatin1::getProperties (11 bytes)
55   19       4       java.lang.String::charAt (29 bytes)
57   15       3       java.lang.StringBuilder::append (8 bytes)
58    1       3       java.lang.String::charAt (29 bytes)   made not entrant
59   26       1       java.lang.Object:: (1 bytes)
59    7       3       java.lang.Object:: (1 bytes)   made not entrant
59   22       3       PrimeNumbers::isPrime (35 bytes)
60   25       3       java.lang.Integer:: (10 bytes)
60   29 %     4       PrimeNumbers::isPrime @ 2 (35 bytes)

Explanation of Specific Lines

Line 1:

44    1       3       java.lang.String::charAt (29 bytes)

• 44: Timestamp in milliseconds since JVM start.
• 1: Compilation ID.
• 3: Optimization level.
• java.lang.String::charAt (29 bytes): Method name and size.

Line 2:

46    2     n 0       java.lang.System::arraycopy (native)   (static)

• 46: Timestamp in milliseconds since JVM start.
• 2: Compilation ID.
• n 0: Native method compilation.
• java.lang.System::arraycopy (native): Method name, indicating it's a native method.
• (static): Indicates that the method is static.

Line 3:

48    3       3       java.lang.String::hashCode (55 bytes)

• 48: Timestamp in milliseconds since JVM start.
• 3: Compilation ID.
• Similar to Line 1, but for the hashCode method.

Line 16:

60   29 %     4       PrimeNumbers::isPrime @ 2 (35 bytes)

• 60: Timestamp in milliseconds since JVM start.
• 29: Compilation ID.
• % 4: Indicates high-level optimization.
• PrimeNumbers::isPrime @ 2 (35 bytes): Method name and size, compiled at high optimization level.

Not Entrant Methods

Not Entrant: Indicates that the method is no longer considered "hot" and is de-optimized, potentially to be recompiled if it becomes hot again.

58    1       3       java.lang.String::charAt (29 bytes)   made not entrant
59    7       3       java.lang.Object:: (1 bytes)   made not entrant

Summary of Compilation Markers

• n Marker: Indicates native methods.
• s Marker: Indicates synchronized methods.
• % Marker: Indicates methods compiled with high-level optimization.

This detailed breakdown should help you understand the output of the -XX:+PrintCompilation flag and the significance of the n, s, and % markers in the context of JIT compilation in the JVM.