Maximum size of a method in Java 7 and 8

Question

I know that a method cannot be larger than 64 KB with Java. The limitation causes us problems with generated code from a JavaCC grammar. We had problems with Java 6 and

Answer 1

According to JVMS7 :

The fact that end_pc is exclusive is a historical mistake in the design of the Java virtual machine: if the Java virtual machine code for a method is exactly 65535 bytes long and ends with an instruction that is 1 byte long, then that instruction cannot be protected by an exception handler. A compiler writer can work around this bug by limiting the maximum size of the generated Java virtual machine code for any method, instance initialization method, or static initializer (the size of any code array) to 65534 bytes.

But this is about Java 7. There is no final specs for Java 8, so nobody (except its developers) could answer this question.

UPD (2015-04-06) According to JVM8 it is also true for Java 8.

Answer 2

Good question. As always we should go to the source to find the answer ("The Java® Virtual Machine Specification"). The section does not explicitly mention a limit (as did the Java6 VM spec) though, but somewhat circumspectly:

The greatest number of local variables in the local variables array of a frame created upon invocation of a method (§2.6) is limited to 65535 by the size of the max_locals item of the Code attribute (§4.7.3) giving the code of the method, and by the 16-bit local variable indexing of the Java Virtual Machine instruction set.

Cheers,

Answer 3

It has not changed. The limit of code in methods is still 64 KB in both Java 7 and Java 8.

References:

1. From the Java 7 Virtual Machine Specification (4.9.1 Static Constraints):

The static constraints on the Java Virtual Machine code in a class file specify how Java Virtual Machine instructions must be laid out in the code array and what the operands of individual instructions must be.

The static constraints on the instructions in the code array are as follows:

• The code array must not be empty, so the code_length item cannot have the value 0.
• The value of the code_length item must be less than 65536.

2. From the Java 8 Virtual Machine Specification (4.7.3 The Code Attribute):

The value of the code_length item gives the number of bytes in the code array for this method.

The value of code_length must be greater than zero (as the code array must not be empty) and less than 65536.

Answer 4

As a workaround, and if you have access to the parser's code, you could modify it to work within whatever 'limits are imposed by the JVM compiler ... (Assuming it den't take forever to find the portions in the parser code to modify)

Answer 5

Andremoniy has answered the java 7 part of this question already, but seems at that time it was soon to decide about java 8 so I complete the answer to cover that part:

Quoting from jvms:

The fact that end_pc is exclusive is a historical mistake in the design of the Java Virtual Machine: if the Java Virtual Machine code for a method is exactly 65535 bytes long and ends with an instruction that is 1 byte long, then that instruction cannot be protected by an exception handler. A compiler writer can work around this bug by limiting the maximum size of the generated Java Virtual Machine code for any method, instance initialization method, or static initializer (the size of any code array) to 65534 bytes.

As you see seems this historical problem doesn't seem to remedy at least in this version (java 8).